Benefit Plans - Summary of Company's Option Activity (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Options, Beginning Balance	1,208,860	1,584,435	1,410,650
Number of Options, granted	0	299,600	881,800
Number of Options, exercised	(332,810)	(398,275)	(368,085)
Number of Options, cancelled/forfeited/expired	(30,450)	(276,900)	(339,930)
Number of Options, Ending Balance	845,600	1,208,860	1,584,435
Number of Options, Exercisable at December 31, 2017	303,225
Number of Options, Expected to Vest after December 31, 2017	517,884
Weighted-Average Price, Beginning Balance	$ 28.14	$ 28.14	$ 29.91
Weighted-Average Price, Options granted		34.66	25.26
Weighted-Average Price, Options exercised	29.49	28.94	27.34
Weighted-Average Price, Options cancelled/forfeited/expired	28.27	28.04	28.62
Weighted-Average Price, Ending Balance	29.57	$ 28.14	$ 28.14
Weighted-Average Price, Exercisable at December 31, 2017	29.71
Weighted-Average Price, Expected to Vest after December 31, 2017	$ 29.50
Weighted-Average Remaining Contractual Term, Balance At December 31, 2017	4 years 2 months 4 days
Weighted-Average Remaining Contractual Term, Exercisable at December 31, 2017	3 years 9 months 10 days
Weighted-Average Remaining Contractual Term, Expected to Vest after December 31, 2017	4 years 4 months 28 days
Aggregate Intrinsic Value, Balance At December 31, 2017	$ 18.6
Aggregate Intrinsic Value, Exercisable at December 31, 2017	6.6
Aggregate Intrinsic Value, Expected to Vest after December 31, 2017	$ 11.4

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

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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 vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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