Income Before Provision for Income Taxes and Related Provision for Income Taxes (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Income before provision for income taxes	$ 13,935	$ 12,666	$ 16,031	$ 18,069	$ 16,523	$ 15,899	$ 15,484	$ 15,320	$ 60,701	$ 63,226	$ 77,822
Current:
Federal, provision for income taxes, current									17,241	21,728	15,391
State, provision for income taxes, current									2,270	2,967	428
Total current									19,511	24,695	15,819
Deferred (prepaid):
Federal, provision for income taxes, deferred									3,027	(200)	(1,713)
State, provision for income taxes, deferred									34	208	(582)
Total deferred (prepaid)									3,061	8	(2,295)
Total provision for income taxes	$ 5,255	$ 4,399	$ 6,031	$ 6,887	$ 6,417	$ 6,123	$ 6,036	$ 6,127	$ 22,572	$ 24,703	$ 13,524