Income Taxes - Components of Income Tax Expense (Benefit) (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									$ 76,966	$ 64,006	$ 63,481
State									8,589	4,831	5,222
Current Income Tax Expense (Benefit), Total									85,555	68,837	68,703
Deferred:
Federal									(6,189)	7,961	4,473
State									843	891	557
Deferred Income Tax Expense (Benefit)									(5,346)	8,852	5,030
Tax expense recorded as an increase of paid-in capital										9,921	3,863
Income Tax Expense (Benefit), Total	$ 23,320	$ 21,266	$ 15,485	$ 20,138	$ 28,421	$ 15,187	$ 17,257	$ 26,745	$ 80,209	$ 87,610	$ 77,596