Derivative Commodity Instruments - Offsetting Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative assets
Gross Amounts Recognized	$ 114,784	$ 149,504
Gross Amounts Offset in the Balance Sheets	(29,697)	(44,135)
Net Amount Presented in the Balance Sheets	85,087	105,369
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	85,087	105,369
Derivative assets
Gross Amounts Recognized	42,203	56,328
Gross Amounts Offset in the Balance Sheets	(29,697)	(44,135)
Net Amount Presented in the Balance Sheets	12,506	12,193
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	$ 12,506	$ 12,193

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

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