Stock Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock based compensation cost recognized	$ 9.0	$ 8.7
Stock option compensation expense recognized	$ 4.5	$ 4.3
Weighted-average fair value per option at the date of grant	$ 8.15	$ 10.83
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years	3 years
Award expiration period	10 years	10 years
Restricted Stock And Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years	3 years
Number of Units - Granted	169,407	136,647
Value of share units at the date of issuance	$ 7.2	$ 6.8
Share based compensation expense attributable to share units	$ 4.5	$ 4.4
A.O. Smith Combined Compensation Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for granting of options, restricted stock or share units	951,461

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Amount of noncash expense for award of restricted stock or unit under share-based payment arrangement.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of noncash expense for option under share-based payment arrangement.

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