Reconciliation of Shares in Computation of Net Income Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income	$ 24,514	$ 32,599	$ 31,144	$ 28,224	$ 29,917	$ 33,496	$ 33,341	$ 27,480	$ 116,481	$ 124,234	$ 103,475
Earnings per share:
Basic	$ 0.36	$ 0.47	$ 0.45	$ 0.40	$ 0.42	$ 0.47	$ 0.46	$ 0.38	$ 1.68	$ 1.73	$ 1.41
Effect of CESs										(0.01)	(0.01)
Diluted	$ 0.36	$ 0.47	$ 0.45	$ 0.40	$ 0.42	$ 0.47	$ 0.46	$ 0.38	$ 1.68	$ 1.72	$ 1.40
Weighted average number of shares:
Basic	68,485	68,928	69,227	69,973	70,742	71,403	71,880	72,630	69,175	71,674	73,443
Effect of CESs									249	386	595
Diluted	68,791	69,135	69,421	70,247	71,148	71,743	72,228	73,020	69,424	72,060	74,038