Derivative Commodity Instruments - Offsetting Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Derivative assets
Gross Amounts Recognized at Fair Value	$ 166,263	$ 339,977
Gross Amounts Offset in the Balance Sheets	(18,474)	(17,640)
Total assets	147,789	322,337
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	147,789	322,337
Derivative assets
Gross Amounts Recognized at Fair Value	32,579	18,628
Gross Amounts Offset in the Balance Sheets	(18,474)	(17,640)
Net Amount Presented in the Balance Sheets	14,105	988
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	14,105	988
Total Derivatives	133,684	321,349
Current Assets
Derivative assets
Gross Amounts Recognized at Fair Value	165,556	339,977
Gross Amounts Offset in the Balance Sheets	(17,767)	(17,640)
Total assets	147,789	322,337
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	147,789	322,337
Noncurrent Assets
Derivative assets
Gross Amounts Recognized at Fair Value	707	0
Gross Amounts Offset in the Balance Sheets	(707)	0
Total assets	0	0
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	0	0
Current Liabilities
Derivative assets
Gross Amounts Recognized at Fair Value	27,987	18,628
Gross Amounts Offset in the Balance Sheets	(17,767)	(17,640)
Net Amount Presented in the Balance Sheets	10,220	988
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	10,220	988
Noncurrent Liabilities
Derivative assets
Gross Amounts Recognized at Fair Value	4,592	0
Gross Amounts Offset in the Balance Sheets	(707)	0
Net Amount Presented in the Balance Sheets	3,885	0
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	$ 3,885	$ 0