Fair Value Measurements (Level 3 Fair Value Measurements) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Fair value of Level 3 instruments, beginning of period	$ 165,015	$ (6,097)	$ 188,446	$ 6,709
Fair value adjustments on commodity derivatives	(7,302)	(33,019)	17,783	(45,825)
Receipt on settlements of commodity derivatives	(45,355)	0	(93,871)	0
Fair value of Level 3 instruments, end of period	112,358	(39,116)	112,358	(39,116)
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets or liabilities still held at the reporting date	$ (4,325)	$ (33,019)	$ 14,142	$ (45,825)