Reconciliation of Net Income and Share Amounts in Computation of Basic and Diluted Net Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income	$ 32,599	$ 33,496	$ 91,967	$ 94,317	$ 124,234
Earnings per share:
Basic	$ 0.47	$ 0.47	$ 1.33	$ 1.31
Effect of CESs			(0.01)	(0.01)
Diluted	$ 0.47	$ 0.47	$ 1.32	$ 1.30
Weighted average number of shares:
Basic	68,928	71,403	69,389	71,981
Effect of CESs	207	340	225	359
Diluted	69,135	71,743	69,614	72,340