Income Taxes - Components of Provision for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current
Federal	$ 15,670	$ 12,790	$ 28,077
State	1,985	1,855	2,539
Total current tax expense (benefit)	17,655	14,645	30,616
Deferred
Federal	20,895	5,489	4,339
State	1,940	910	2,017
Total deferred tax expense (benefit)	22,835	6,399	6,356
Income tax expense (benefit)	$ 40,490	$ 21,044	$ 36,972