Fair Value Measurements (Details) - USD ($)	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Derivative Asset, Fair Value, Net [Abstract]
Derivative Current Asset, Fair Value	$ 134,400,000	$ 214,800,000
Derivative, Fair Value, Net [Abstract]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	0		$ 0
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset (Liability)	134,235,000	214,585,000
Derivative Asset (Liability), Fair Value, Gross Liability (Asset)	0	0
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	134,235,000	214,585,000
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Other Current Assets [Member]
Derivative Asset, Fair Value, Net [Abstract]
Derivative Current Asset, Fair Value, Gross Asset	123,669,000	183,625,000
Derivative Current Asset, Fair Value, Gross Liability	(9,487,000)	(12,524,000)
Derivative Current Asset, Fair Value	114,182,000	171,101,000
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Other Noncurrent Assets [Member]
Derivative Asset, Fair Value, Net [Abstract]
Derivative Current Asset, Fair Value, Gross Asset	20,289,000	44,725,000
Derivative Current Asset, Fair Value, Gross Liability	(118,000)	(1,041,000)
Derivative Current Asset, Fair Value	20,171,000	43,684,000
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Derivative Liabilities Current [Member]
Derivative Liability, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	(9,605,000)	(12,707,000)
Derivative Liability, Fair Value, Gross Asset	9,487,000	12,524,000
Derivative Liability	(118,000)	(183,000)
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Other Noncurrent Liabilities [Member]
Derivative Liability, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	(118,000)	(1,058,000)
Derivative Liability, Fair Value, Gross Asset	118,000	1,041,000
Derivative Liability	$ 0	$ (17,000)