Earnings Per Share - Computation of Basic and Diluted Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share Basic And Diluted [Abstract]
Net income	$ 148,089	$ 126,172	$ 96,037
Basic weighted average shares outstanding	36,864	36,844	36,690
Dilutive effect of stock options and restricted stock	1,174	894	947
Diluted weighted average shares outstanding	38,038	37,738	37,637
Basic earnings per share	$ 4.02	$ 3.42	$ 2.62
Diluted earnings per share	$ 3.89	$ 3.34	$ 2.55