Income Taxes (Schedule of Effective Tax Rate Reconciliation) (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Reconciliation, Amount
Income tax provision (benefit) calculated using the federal statutory income tax rate	$ 86,086	$ 16,275	$ (532,121)
State income taxes, net of federal income tax benefit	11,968	2,764	(25,351)
Tax shortfall (windfall) on stock-based compensation deduction	(1,565)	5,567	9,557
Valuation allowance	(42)	5,562	2,910
Enhanced oil recovery credits generated	(10,818)	(11,307)	0
Remeasurement of deferreds related to federal tax rate change	0	(132,224)	0
Other	1,604	(3,289)	835
Total income tax expense (benefit)	$ 87,233	$ (116,652)	$ (544,170)