Income Taxes - Income Tax (Benefit) Expense Attributable to Income Before Income Taxes (Details) - 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
Components Of Income Tax Expense Benefit Continuing Operations [Abstract]
U.S. federal, Current									$ 0	$ 0	$ 0
U.S. state and local, Current									17	98	29
Total, Current	$ 17	$ 0	$ 0	$ 0	$ 98	$ 0	$ 0	$ 0	17	98	29
U.S. federal, Deferred									(302,507)	(266,105)	(328,257)
U.S. state and local, Deferred									51,464	(14,743)	(10,449)
Total, Deferred	(349,097)	(71,992)	57,651	112,395	(95,679)	(13,705)	(129,488)	(41,976)	(251,043)	(280,848)	(338,706)
U.S. federal, Total									(302,507)	(266,105)	(328,257)
U.S. state and local, Total									51,481	(14,645)	(10,420)
Total (benefit) expense for income taxes	$ (349,080)	$ (71,992)	$ 57,651	$ 112,395	$ (95,581)	$ (13,705)	$ (129,488)	$ (41,976)	$ (251,026)	$ (280,750)	$ (338,677)