Derivative Commodity Instruments - Offsetting Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Derivative assets
Gross Amounts Recognized at Fair Value	$ 179,548	$ 339,977
Gross Amounts Offset in the Balance Sheets	(23,215)	(17,640)
Total assets	156,333	322,337
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	156,333	322,337
Derivative assets
Gross Amounts Recognized at Fair Value	29,218	18,628
Gross Amounts Offset in the Balance Sheets	(23,215)	(17,640)
Net Amount Presented in the Balance Sheets	6,003	988
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	6,003	988
Total Derivatives	150,330	321,349
Current Assets
Derivative assets
Gross Amounts Recognized at Fair Value	176,061	339,977
Gross Amounts Offset in the Balance Sheets	(22,245)	(17,640)
Total assets	153,816	322,337
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	153,816	322,337
Noncurrent Assets
Derivative assets
Gross Amounts Recognized at Fair Value	3,487	0
Gross Amounts Offset in the Balance Sheets	(970)	0
Total assets	2,517	0
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	2,517	0
Current Liabilities
Derivative assets
Gross Amounts Recognized at Fair Value	25,324	18,628
Gross Amounts Offset in the Balance Sheets	(22,245)	(17,640)
Net Amount Presented in the Balance Sheets	3,079	988
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	3,079	988
Noncurrent Liabilities
Derivative assets
Gross Amounts Recognized at Fair Value	3,894	0
Gross Amounts Offset in the Balance Sheets	(970)	0
Net Amount Presented in the Balance Sheets	2,924	0
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	$ 2,924	$ 0