Reconciliation of Net Income and Shares Amounts in Computation of Basic and Diluted Net Earnings Per Common Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Net income	$ 20,354	$ 22,340	$ 20,594	$ 18,712	$ 16,846	$ 19,688	$ 17,407	$ 13,355	$ 82,000	$ 67,296	$ 51,853
Earnings per share:
Basic	$ 0.27	$ 0.30	$ 0.27	$ 0.25	$ 0.22	$ 0.26	$ 0.23	$ 0.17	$ 1.09	$ 0.88	$ 0.66
Effect of CESs									$ (0.01)	$ (0.02)	$ (0.02)
Diluted	$ 0.27	$ 0.30	$ 0.27	$ 0.24	$ 0.22	$ 0.25	$ 0.22	$ 0.17	$ 1.08	$ 0.86	$ 0.64
Weighted average number of shares:
Basic	74,223	74,687	75,274	75,817	76,032	76,452	76,888	77,308	74,995	76,664	78,640
Effect of CESs									846	1,268	2,444
Diluted	75,034	75,466	76,037	76,795	77,256	77,552	78,036	78,740	75,841	77,932	81,084