Accounting for Stock Based Compensation - Summarizes Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock Options Outstanding, beginning balance, Shares	869,000	1,653,000	2,652,142
Granted, Shares	0	0	0
Exercised, Shares	(434,250)	(428,250)	(615,358)
Forfeited, Shares	(6,750)	(355,750)	(383,784)
Stock Options Outstanding, ending balance, Shares	428,000	869,000	1,653,000	2,652,142
Stock Options Exercisable, Shares			338,625
Stock Options Outstanding, beginning balance, Weighted-Average Exercise Price	$ 37.87	$ 38.53	$ 35.34
Exercised, Weighted-Average Exercise Price	38.12	38.66	24.01
Forfeited, Weighted-Average Exercise Price	38.05	39.99	39.59
Stock Options Outstanding, ending balance, Weighted-Average Exercise Price	$ 37.61	$ 37.87	38.53	$ 35.34
Exercisable, Weighted-Average Exercise Price			$ 37.87
Stock Options Outstanding, Weighted-Average Remaining Contractual Term	3 years 2 months 12 days	3 years 6 months 3 days	4 years 3 months 10 days	4 years 9 months 3 days
Exercisable, Weighted-Average Remaining Contractual Term			3 years 1 month 9 days
Stock Options Outstanding, Aggregate Intrinsic Value	$ 11,815	$ 5,172	$ 3,610	$ 6,964
Exercisable, Aggregate Intrinsic Value			$ 9,261