Income Taxes (Schedule of Effective Tax Rate Reconciliation) (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Amount
Income tax provision (benefit) calculated using the federal statutory income tax rate	$ 16,275	$ (532,121)	$ (2,214,094)
State income taxes, net of federal income tax benefit	2,764	(25,351)	(117,624)
Impairment of goodwill with no related tax basis	0	0	363,666
Tax shortfall on stock-based compensation deduction	5,567	9,557	0
Valuation allowance	5,562	2,910	33,600
Enhanced oil recovery credits generated	(11,307)	0	0
Remeasurement of deferreds related to federal tax rate change	(132,224)	0	0
Other	(3,289)	835	(6,082)
Total income tax benefit	$ (116,652)	$ (544,170)	$ (1,940,534)