Income Taxes - Provision for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Current federal income tax (provision) benefit	$ 7,781	$ 22,941	$ 0
Current tax (provision), various states	0	(2)	(24)
Total current income tax (provision) benefit	7,781	22,939	(24)
Deferred federal income tax (provision) benefit	(81,054)	182,422	140,578
Tax benefit from US tax reform legislation	713,655	0	0
Deferred tax (provision) benefit, various states	(7,002)	27,414	40,863
Total deferred income tax benefit	625,599	209,836	181,441
Benefit for income taxes	$ 633,380	$ 232,775	$ 181,417