Benefit Plans - Summary of Company's Option Activity (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Options, Beginning Balance	580,140	845,600	1,208,860
Number of Options, granted	0	0	0
Number of Options, exercised	(163,670)	(260,860)	(332,810)
Number of Options, cancelled/forfeited/expired	(7,060)	(4,600)	(30,450)
Number of Options, Ending Balance	409,410	580,140	845,600
Number of Options, Exercisable at December 31, 2020	309,695
Number of Options, Expected to Vest after December 31, 2020	97,645
Weighted-Average Price, Beginning Balance	$ 29.57	$ 28.14	$ 28.14
Weighted-Average Price, Options exercised	30.22	29.55	29.49
Weighted-Average Price, Options cancelled/forfeited/expired	28.95	30.59	28.27
Weighted-Average Price, Ending Balance	29.33	$ 29.57	$ 28.14
Weighted-Average Price, Exercisable at December 31, 2020	28.45
Weighted-Average Price, Expected to Vest after December 31, 2020	$ 32.05
Weighted-Average Remaining Contractual Term, Balance At December 31, 2020	2 years 6 months 7 days
Weighted-Average Remaining Contractual Term, Exercisable at December 31, 2020	2 years 4 months 24 days
Weighted-Average Remaining Contractual Term, Expected to Vest after December 31, 2020	2 years 11 months 12 days
Aggregate Intrinsic Value, Balance At December 31, 2020	$ 15.5
Aggregate Intrinsic Value, Exercisable at December 31, 2020	12.0
Aggregate Intrinsic Value, Expected to Vest after December 31, 2020	$ 3.4

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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