Accounting Policies [Abstract] Accounting Policies [Abstract] Accounts Payable, Trade, Current Trade payables Accrued Liabilities, Current Accrued liabilities Accrued Liabilities [Member] Accrued Liabilities Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Pension liability Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member] Prior year service cost Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member] Actuarial losses Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Less accumulated depreciation and amortization Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] Unrealized net gain on cash flow derivatives Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax Unrealized net (loss) gain on cash flow derivative instruments less related income tax benefit (provision) of $0.6 in 2017 and $(0.1) in 2016 Accumulated other comprehensive income loss cumulative changes in net gain loss from cash flow hedges effects tax. Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effects Tax Unrealized net (loss) gain on cash flow derivative instruments, related income tax benefit (provision) Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax Pension liability less related income tax benefit of $175.9 in 2017 and $183.4 in 2016 Accumulated other comprehensive income loss defined benefit pension and other postretirement plans tax effect. Accumulated Other Comprehensive Income Loss Defined Benefit Pension and Other Postretirement Plans Tax Effect Pension liability, related income tax benefit Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax Cumulative foreign currency translation adjustments Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss), Net of Tax Accumulated other comprehensive loss Accumulated other comprehensive loss Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract] Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract] Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] AOCI Attributable to Parent [Member] Accumulated Other Comprehensive Loss Accumulated Foreign Currency Adjustment Attributable to Parent [Member] Cumulative foreign currency translation Acquired Finite-lived Intangible Assets, Weighted Average Useful Life Acquired intangible assets, amortization period, in years Amortization of intangible assets, weighted average period, years Additional Paid in Capital, Common Stock Capital in excess of par value Additional Paid-in Capital [Member] Capital in Excess of Par Value Adjustments, Noncash Items, to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Adjustments to reconcile earnings to cash provided by (used in) operating activities: Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition Stock based compensation expense Advertising Costs, Policy [Policy Text Block] Advertising Advertising Expense Advertising costs charged to operations All Currencies [Domain] All Currencies [Domain] Allocated Share-based Compensation Expense Stock based compensation expense Total stock based compensation cost recognized All Other [Member] All Other [Member] Other Allowance for Doubtful Accounts Receivable Allowance for doubtful accounts Amendment Flag Amendment Flag Amortization associated with prepaid advertising costs. Amortization Of Advertising Costs Amortization of total advertising costs Amortization of Intangible Assets Amortization expenses of other intangible assets Amortization of total advertising costs in months Amortization Period of Advertising Costs Amortization of total advertising costs, months Defined Benefit Plan, Amount Recognized in Net Periodic Benefit Cost (Credit) and Other Comprehensive (Income) Loss, before Tax Total recognized in net periodic (benefit) cost and other comprehensive loss Annually [Member] Annually [Member] Annually A.O.Smith Combined Compensation Plan [Member] A- O- Smith Combined Compensation Plan [Member] A.O. Smith Combined Compensation Plan Aquasana [Member] Aquasana [Member] Aquasana Assets Total Assets Total Assets Assets [Abstract] Assets Assets, Current Total Current Assets Assets, Fair Value Disclosure Assets measured at fair value on recurring basis Award Date [Axis] Award Date [Axis] Award Date [Domain] Award Date [Domain] Award Type [Axis] Award Type [Axis] Balance Sheet Location [Axis] Balance Sheet Location Balance Sheet Location [Domain] Balance Sheet Location [Domain] Bank Credit Lines [Member] Bank Credit Lines [Member] Bank Credit Lines Building [Member] Building Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Business Acquisition [Axis] Business Acquisition Business Acquisition [Line Items] Business Acquisition [Line Items] Business Acquisition, Percentage of Voting Interests Acquired Percentage of ownership interest acquired Business Combination, Acquisition Related Costs Acquisition-related costs Business combination, contingent consideration arrangements, conditions measurement period. Business Combination, Contingent Consideration Arrangements, Conditions Measurement Period Contingent consideration, conditions measurement period Business Combination, Contingent Consideration, Liability Contingent consideration and holdback Business Combination Contingent Consideration Paid Business Combination Contingent Consideration Paid Acquisition related contingent payment Business Combination Disclosure [Text Block] Acquisitions Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual Pre tax earnings Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual Revenues Business combination recognized identifiable assets acquired and liabilities assumed assets including goodwill. Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Assets Including Goodwill Total assets acquired Business combination, recognized identifiable assets acquired and liabilities assumed, current assets net of cash acquired. Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets Net of Cash Acquired Current assets, net of cash acquired Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities Current liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets Acquired intangible assets, not subject to amortization Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill Acquired intangible assets Intangible assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles Acquired intangible assets, subject to amortization Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities Total liabilities assumed Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities Long-term liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment Property, plant and equipment Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net Net assets acquired Business Combinations [Abstract] Business Combinations [Abstract] CANADA Canada Canada, Dollars Canadian dollar Canadian Term Notes With Insurance Companies [Member] Canadian Term Notes With Insurance Companies [Member] Canadian Term Notes With Insurance Companies Cash Acquired from Acquisition Cash acquired from business acquisition Cash And Cash Equivalents And Marketable Securities Were Held By Foreign Subsidiaries Cash And Cash Equivalents And Marketable Securities Were Held By Foreign Subsidiaries Cash and cash equivalents and marketable securities were held by foreign subsidiaries Cash and Cash Equivalents, at Carrying Value Cash and Cash Equivalents-End of Year Cash and cash equivalents-beginning of year Cash and cash equivalents Cash and Cash Equivalents, Period Increase (Decrease) Net increase in cash and cash equivalents Cash and Cash Equivalents, Policy [Policy Text Block] Cash and cash equivalents Cash Flow Supplemental Disclosures [Line Items] Cash Flow Supplemental Disclosures [Line Items] Cash Flow Supplemental Disclosures [Line Items] Cash Flow Supplemental Disclosures [Table] Cash Flow Supplemental Disclosures [Table] Cash Flow Supplemental Disclosures [Table] Cash Flow, Supplemental Disclosures [Text Block] Statement of Cash Flows Class of Stock [Domain] Class of Stock [Domain] CHINA China Commercial Paper [Member] Commercial Paper Commingled Equity Funds [Member] Commingled Equity Funds [Member] Commingled Equity Funds Commingled Fixed Income Funds [Member] Commingled Fixed Income Funds [Member] Commingled Fixed Income Funds Commitments and Contingencies Commitments and contingencies Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Text Block] Commitments and Contingencies Commitments and Contingencies [Line Items] Commitments and Contingencies [Line Items] Commitments and Contingencies [Line Items] Commitments and Contingencies [Table] Commitments and Contingencies [Table] Commitments and Contingencies [Table] Commodity Contract [Member] Commodities Contracts Commodity Futures Contracts [Member] Commodity Futures Contracts [Member] Commodity Futures Contracts Common Class A [Member] Common Class A Common Stock Class Undefined [Member] Common Stock Class Undefined [Member] Common Stock Common Stock, Dividends, Per Share, Cash Paid Dividends paid Common Stock, Dividends, Per Share, Declared Common dividends declared Common Stock [Member] Common Stock Common Stock, Par or Stated Value Per Share Common stock, par value Common Stock, Shares Authorized Common Stock, shares authorized Common Stock, Shares, Issued Common Stock, shares issued Common Stock, Value, Issued Common Stock, value Retirement Benefits [Abstract] Retirement Benefits [Abstract] Component Of Net Income Tax Assets And Liabilities [Line Items] Component Of Net Income Tax Assets And Liabilities [Line Items] Component Of Net Income Tax Assets And Liabilities [Line Items] Component Of Net Income Tax Assets And Liabilities [Table] Component Of Net Income Tax Assets And Liabilities [Table] Component Of Net Income Tax Assets And Liabilities [Table] Compounded annualized return of pension plan. Compounded Annualized Return Of Pension Plan Compounded annualized return of pension plan Comprehensive Income (Loss), Net of Tax, Attributable to Parent Comprehensive Earnings Computation of Earnings Per Share [Line Items] Computation of Earnings Per Share [Line Items] Computation of Earnings Per Share [Line Items] Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Concentration Risk Type [Axis] Concentration Risk Type [Axis] Concentration Risk, Percentage Concentration Risk, Percentage Concentration Risk Type [Domain] Concentration Risk Type [Domain] Condensed Financial Information of Subsidiaries Disclosure [Abstract] Condensed Financial Information of Subsidiaries Disclosure [Abstract] Condensed Financial Information of Subsidiaries Disclosure [Abstract] Consolidation Items [Axis] Consolidation Items [Axis] Consolidation Items [Domain] Consolidation Items [Domain] Consolidation, Policy [Policy Text Block] Consolidation Conversion of Stock, Shares Converted Shares converted into Common Stock Corporate, Non-Segment [Member] Corporate Cost of Goods Sold Cost of products sold Cost of Sales [Member] Cost Of Products Sold Credit Facility [Axis] Credit Facility [Axis] Credit Facility [Domain] Credit Facility [Domain] Currency [Axis] Currency [Axis] Current Federal Tax Expense (Benefit) Current, Federal Current Fiscal Year End Date Current Fiscal Year End Date Current Foreign Tax Expense (Benefit) Current, International Current State and Local Tax Expense (Benefit) Current, State Customer Lists [Member] Customer Lists Customer one Customer 1 [Member] Customer one Customer Relationships [Member] Customer Relationships Customer two Customer 2 [Member] Customer Two Debt Disclosure [Abstract] Debt Disclosure [Abstract] Debt Disclosure [Text Block] Debt and Lease Commitments Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Face Amount Debt instrument, issued Debt Instrument, First Installment Payment, Year Debt Instrument, First Installment Payment, Year Debt instrument, principle repayment commencement year Debt Instrument, Interest Rate, Stated Percentage Debt instrument, interest rate Debt Instrument [Line Items] Debt Instrument [Line Items] Debt Instrument Maturity Date Year Debt Instrument Maturity Date Year Debt instrument maturity Debt instrument, maturity year Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Debt Instrument, Periodic Payment, Interest Interest paid Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Debt Instrument Tranche One [Member] Debt Instrument Tranche One [Member] Tranche One Debt Instrument Tranche Two [Member] Debt Instrument Tranche Two [Member] Tranche Two Debt Securities [Member] Debt Securities Decrease in Unrecognized Tax Benefits is Reasonably Possible Anticipated decrease in unrecognized tax benefits Deferred Federal Income Tax Expense (Benefit) Deferred, Federal Deferred Foreign Income Tax Expense (Benefit) Deferred, International Deferred Tax Liabilities, Gross All other, Liabilities Deferred State and Local Income Tax Expense (Benefit) Deferred, State The tax effect as of the balance sheet date of the amount of the estimated future tax deductions arising from estimated environmental reserves, which can only be deducted for tax purposes when environmental losses are actually incurred, and which can only be realized if sufficient tax-basis income is generated in future periods to enable the deduction to be taken. Deferred Tax Asset Environmental Environmental, Assets Deferred Tax Assets, Accounts Receivable Deferred Tax Assets Accounts Receivable Accounts receivable, Assets Deferred Tax Assets Goodwill And Intangible Assets Intangible Assets Deferred Tax Assets Goodwill And Intangible Assets Intangible Assets Intangibles, Assets Deferred Tax Assets, Inventory Inventories, Assets Deferred Tax Assets, Net Net asset Deferred Tax Assets, Net of Valuation Allowance Assets Deferred Tax Assets, Other All other, Assets Deferred Tax Assets, Property, Plant and Equipment Property, plant and equipment, Assets Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Benefits Employee benefits, Assets Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Warranty Reserves Product liability and warranties, Assets Deferred Tax Assets, Undistributed Foreign Earnings Deferred Tax Assets, Undistributed Foreign Earnings Undistributed foreign earnings, Assets Deferred Tax Assets, Valuation Allowance Balance at End of Year Balance at Beginning of Year Valuation allowance, Assets The sum of the tax effects as of the balance sheet date of the amount of excesses of tax deductions over gross income in a year which cannot be used on the tax returns in the current year but can be carried forward to reduce taxable income or income taxes payable in a future year, for which there must be sufficient tax-basis income to utilize a portion or all of the carryforward amount to realize the deferred tax asset and the tax effect as of the balance sheet date of the amount of future tax deductions arising from all unused tax credit carryforwards which have been reduced by a valuation allowance. Deferred Tax Asset Tax Loss And Credit Carryovers Tax loss and credit carryovers, Assets Deferred Tax Liabilities Accounts Receivable Deferred Tax Liabilities Accounts Receivable Accounts receivable, Liabilities Deferred Tax Liabilities, Environmental Trust Funds Deferred Tax Liabilities, Environmental Trust Funds Environmental, Liabilities Deferred Tax Liabilities, Intangible Assets Intangibles, Liabilities The amount as of the balance sheet date of the estimated future tax effects attributable to LIFO reserve calculation differences between generally accepted accounting principles and tax purposes and the inventory related costs capitalized for tax purposes but expensed in conformity with generally accepted accounting principles, which will reverse in future periods when inventory is sold. Deferred Tax Liabilities Inventories Inventories, Liabilities Deferred Tax Liabilities, Other All other, Liabilities Deferred Tax Liabilities, Property, Plant and Equipment Property, plant and equipment, Liabilities Deferred Tax Liabilities Tax Deferred Expense Compensation And Benefits Employee Compensation Deferred Tax Liabilities Tax Deferred Expense Compensation And Benefits Employee Compensation Employee benefits, Liabilities Deferred Tax Liabilities Tax Deferred Expense Reserves and Accruals Warranty Reserves Deferred Tax Liabilities Tax Deferred Expense Reserves and Accruals Warranty Reserves Product liability and warranties, Liabilities Deferred Tax Liabilities, Undistributed Foreign Earnings Undistributed foreign earnings, Liabilities Deferred Tax Liabilities, Valuation Allowance Deferred Tax Liabilities, Valuation Allowance Valuation allowance, Liabilities Deferred Tax Liability Tax Loss And Credit Carryovers Deferred Tax Liability Tax Loss And Credit Carryovers Tax loss and credit carryovers, Liabilities Liability, Defined Benefit Pension Plan, Noncurrent Pension liabilities Defined Benefit Plan [Abstract] Pension And Other Post-Retirement Benefits Defined Benefit Plan, Accumulated Benefit Obligation Accumulated benefit obligation (ABO) at December 31 Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax Total recognized in accumulated other comprehensive loss Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Gain (Loss), before Tax Net actuarial loss (gain) Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, Prior Service Cost (Credit), before Tax Prior service cost Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss) Actual return on plan assets Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Sold Relating to assets sold during the period Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held Relating to assets still held at the reporting date Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) Actuarial loss including assumption changes Defined Benefit Plan, Amortization of Gain (Loss) Net actuarial loss (gain) Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Prior service cost Defined Benefit Plan Amounts Recognized in Other Comprehensive Income Loss Amortization of Net Gain Loss before Tax Defined Benefit Plan Amounts Recognized in Other Comprehensive Income Loss Amortization of Net Gain Loss before Tax Amortization of net actuarial (loss) gain Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), after Reclassification Adjustment, before Tax Prior service cost Defined Benefit Plan And Defined Contribution Plan Periodic Benefit Cost Defined Benefit Plan And Defined Contribution Plan Periodic Benefit Cost Defined Benefit Plan And Defined Contribution Plan Periodic Benefit Cost, Total Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate Discount rate Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase Average salary increases Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate Discount rate Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets Expected long-term return on plan assets