Stock Based Compensation - Stock Based Compensation Stock Option Activity (Detail) - Employee Stock Option - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of options, outstanding, beginning balance (in shares)	811,902
Number of options, granted (in shares)	0
Number of options, exercised (in shares)	(552,339)
Number of options, forfeited (in shares)	0
Number of options, outstanding, ending balance (in shares)	259,563	811,902
Number of options, vested and exercisable at December 31, 2017 (in shares)	259,563
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-average exercise price, outstanding, beginning balance (in dollars per share)	$ 16.31
Weighted-average exercise price, granted (in dollars per share)	0.00
Weighted-average exercise price, exercised (in dollars per share)	15.50
Weighted-average exercise price, forfeited (in dollars per share)	0.00
Weighted-average exercise price, outstanding, ending balance (in dollars per share)	18.03	$ 16.31
Weighted average exercise price, vested and exercisable at December 31, 2017 (in dollars per share)	$ 18.03
Aggregate intrinsic value, outstanding, beginning balance	$ 27,718
Aggregate intrinsic value, outstanding, ending balance	$ 10,985	$ 27,718
Weighted-average remaining contractual life, outstanding, ending balance	2 years 9 months 5 days	2 years 11 months 16 days
Aggregate intrinsic value, vested and exercisable at December 31, 2017	$ 10,985
Weighted-average remaining contractual life, vested and exercisable at December 31, 2017	2 years 9 months 5 days