Income Taxes - Provision for Federal and State Income Taxes (Detail) - USD ($) $ 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
Current:
Federal									$ 9,273	$ 2,400	$ 5,113
State									961	539	829
Deferred:
Federal									7,350	909	3,877
State									133	33	(61)
Provision for federal and state income taxes, total	$ 5,162	$ 4,416	$ 4,217	$ 3,922	$ 568	$ (1,533)	$ 2,046	$ 2,800	$ 17,717	$ 3,881	$ 9,758