Derivative Financial Instruments - Offsetting Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Offsetting Derivative Assets:
Gross Amounts of Recognized Assets	$ 136,664	$ 223,778
Gross Amounts Offset in the Consolidated Balance Sheets	(54,108)	(70,679)
Net Amounts Presented in the Consolidated Balance Sheets	82,556	153,099
Offsetting Derivative Liabilities:
Gross Amounts of Recognized Liabilities	(55,093)	(72,759)
Gross Amounts Offset in the Consolidated Balance Sheets	54,108	70,679
Net Amounts Presented in the Consolidated Balance Sheets	(985)	(2,080)
Commodity contract
Offsetting Derivative Assets:
Gross Amounts of Recognized Assets	136,664	223,778
Gross Amounts Offset in the Consolidated Balance Sheets	(54,108)	(70,679)
Net Amounts Presented in the Consolidated Balance Sheets	82,556	153,099
Offsetting Derivative Liabilities:
Gross Amounts of Recognized Liabilities	(54,108)	(70,679)
Gross Amounts Offset in the Consolidated Balance Sheets	54,108	70,679
Net Amounts Presented in the Consolidated Balance Sheets	0	0
Interest rate contract
Offsetting Derivative Liabilities:
Gross Amounts of Recognized Liabilities	(985)	(2,080)
Gross Amounts Offset in the Consolidated Balance Sheets	0	0
Net Amounts Presented in the Consolidated Balance Sheets	$ (985)	$ (2,080)