Derivative Commodity Instruments - Offsetting Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Derivative assets
Gross Amounts Recognized	$ 22,511	$ 44,215
Gross Amounts Offset in the Balance Sheets	(16,516)	(21,313)
Net Amount Presented in the Balance Sheets	5,995	22,902
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	5,995	22,902
Derivative assets
Gross Amounts Recognized	69,920	51,615
Gross Amounts Offset in the Balance Sheets	(16,516)	(21,313)
Net Amount Presented in the Balance Sheets	53,404	30,302
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	$ 53,404	$ 30,302