Income Taxes - Components of Income Tax Expense Benefit (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
Federal									$ 18,030	$ (12,754)	$ 6,259
State									108	169	96
Total current									18,138	(12,585)	6,355
Deferred:
Federal									(4,568)	35,440	5,798
State									0	0	0
Total deferred									(4,568)	35,440	5,798
Income tax expense (benefit)	$ 4,269	$ 5,278	$ 3,018	$ 1,005	$ 10,186	$ 5,700	$ 3,170	$ 3,799	$ 13,570	$ 22,855	$ 12,153