Derivative Commodity Instruments - Offsetting Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative assets
Gross Amounts Recognized	$ 79,728	$ 149,504
Gross Amounts Offset in the Balance Sheets	(42,839)	(44,135)
Net Amount Presented in the Balance Sheets	36,889	105,369
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	36,889	105,369
Derivative assets
Gross Amounts Recognized	83,987	56,328
Gross Amounts Offset in the Balance Sheets	(42,839)	(44,135)
Net Amount Presented in the Balance Sheets	41,148	12,193
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	$ 41,148	$ 12,193