Derivative Commodity Instruments - Offsetting Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Derivative assets
Gross Amounts Recognized at Fair Value	$ 16,616	$ 44,215
Gross Amounts Offset in the Balance Sheets	(15,469)	(21,313)
Net Amount Presented in the Balance Sheets	1,147	22,902
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	1,147	22,902
Derivative assets
Gross Amounts Recognized at Fair Value	133,387	51,615
Gross Amounts Offset in the Balance Sheets	(15,469)	(21,313)
Net Amount Presented in the Balance Sheets	117,918	30,302
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	117,918	30,302
Total Derivatives	(116,771)	(7,400)
Current Assets
Derivative assets
Gross Amounts Recognized at Fair Value	14,540	36,223
Gross Amounts Offset in the Balance Sheets	(14,016)	(18,760)
Net Amount Presented in the Balance Sheets	524	17,463
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	524	17,463
Noncurrent Assets
Derivative assets
Gross Amounts Recognized at Fair Value	2,076	7,992
Gross Amounts Offset in the Balance Sheets	(1,453)	(2,553)
Net Amount Presented in the Balance Sheets	623	5,439
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	623	5,439
Current Liabilities
Derivative assets
Gross Amounts Recognized at Fair Value	110,229	49,062
Gross Amounts Offset in the Balance Sheets	(14,016)	(18,760)
Net Amount Presented in the Balance Sheets	96,213	30,302
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	96,213	30,302
Noncurrent Liabilities
Derivative assets
Gross Amounts Recognized at Fair Value	23,158	2,553
Gross Amounts Offset in the Balance Sheets	(1,453)	(2,553)
Net Amount Presented in the Balance Sheets	21,705	0
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	$ 21,705	$ 0

Amount of obligation to return cash collateral under master netting arrangements offset against derivative assets.

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