Income Taxes - Components of Income Tax Expense (Benefit) (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current:
Federal									$ 64,006	$ 63,481	$ 50,980
State									4,831	5,222	6,216
Current Income Tax Expense (Benefit), Total									68,837	68,703	57,196
Deferred:
Federal									7,961	4,473	2,734
State									891	557	418
Deferred Income Tax Expense (Benefit)									8,852	5,030	3,152
Tax expense recorded as an increase of paid-in capital									9,921	3,863	7,884
Income Tax Expense (Benefit), Total	$ 28,421	$ 15,187	$ 17,257	$ 26,745	$ 23,916	$ 15,530	$ 16,461	$ 21,689	$ 87,610	$ 77,596	$ 68,232