Stock-Based Compensation Plans - Summary of Stock Options Issued Pursuant to Stock Incentive Plans (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Shares
Outstanding, Beginning Balance	6,791,375	5,799,076	5,282,693
Granted	864,800	2,284,500	2,055,514
Exercised	(1,204,559)	(792,841)	(1,144,273)
Forfeited	(268,417)	(446,845)	(394,265)
Expired	(20,482)	(52,515)	(593)
Outstanding, Ending Balance	6,162,717	6,791,375	5,799,076
Number of Shares Exercisable, Ending Balance	3,486,892
Weighted-Average Exercise Price
Beginning Balance	$ 15.54	$ 14.37	$ 12.06
Granted	20.12	17.92	19.12
Exercised	11.52	11.69	10.62
Forfeited	18.43	18.69	19.06
Expired	20.11	20.44	20.51
Ending Balance	16.83	$ 15.54	$ 14.37
Weighted-Average Exercise Price Exercisable, Ending Balance	$ 15.28
Weighted-Average Remaining Contractual Term (Years)
Weighted Average Remaining Contractual Term (Years), Outstanding as of end of period	6 years 8 months 9 days
Weighted-Average Remaining Contractual Term (Years), Exercisable as of end of period	5 years 6 months 7 days
Aggregate Intrinsic Value of In-the-Money Options
Aggregate Intrinsic Value of In-the-Money Options, Outstanding as of end of period	$ 35,976,295
Aggregate Intrinsic Value of In-the-Money Options, Exercisable as of end of period	$ 25,770,089