Derivative Commodity Instruments - Offsetting Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivative assets
Gross Amounts Recognized at Fair Value	$ 288,071	$ 339,977
Gross Amounts Offset in the Balance Sheets	(15,396)	(17,640)
Total assets	272,675us-gaap_DerivativeAssets	322,337us-gaap_DerivativeAssets
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	272,675	322,337
Derivative assets
Gross Amounts Recognized at Fair Value	29,239	18,628
Gross Amounts Offset in the Balance Sheets	(15,396)	(17,640)
Net Amount Presented in the Balance Sheets	13,843us-gaap_DerivativeLiabilities	988us-gaap_DerivativeLiabilities
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	13,843	988
Total Derivatives	258,832	321,349
Current Assets
Derivative assets
Gross Amounts Recognized at Fair Value	288,062	339,977
Gross Amounts Offset in the Balance Sheets	(15,387)	(17,640)
Total assets	272,675	322,337
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	272,675	322,337
Noncurrent Assets
Derivative assets
Gross Amounts Recognized at Fair Value	9	0
Gross Amounts Offset in the Balance Sheets	(9)	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	28,257	18,628
Gross Amounts Offset in the Balance Sheets	(15,387)	(17,640)
Net Amount Presented in the Balance Sheets	12,870	988
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	12,870	988
Noncurrent Liabilities
Derivative assets
Gross Amounts Recognized at Fair Value	982	0
Gross Amounts Offset in the Balance Sheets	(9)	0
Net Amount Presented in the Balance Sheets	973	0
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	$ 973	$ 0

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

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