Income Taxes - Components of Provision for 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 provision:
Federal									$ 248,191	$ 228,505	$ 215,950
State									13,092	24,336	26,057
Total current provision									261,283	252,841	242,007
Deferred (benefit)/provision:
Federal									(58,124)	(89,518)	(69,546)
State									(628)	786	(7,681)
Total deferred benefit									(58,752)	(88,732)	(77,227)
Provision for income tax expense	$ 66,190	$ 40,617	$ 44,713	$ 51,011	$ 43,122	$ 47,557	$ 34,555	$ 38,875	$ 202,531	$ 164,109	$ 164,780