Derivative Commodity Instruments (Offsetting of Derivative Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets
Gross Amounts Recognized at Fair Value	$ 72,563	$ 339,977
Gross Amounts Offset in the Balance Sheets	(15,600)	(17,640)
Net Amount Presented in the Balance Sheets	56,963	322,337
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	56,963	322,337
Liabilities
Gross Amounts Recognized at Fair Value	16,059	18,628
Gross Amounts Offset in the Balance Sheets	(15,600)	(17,640)
Net Amount Presented in the Balance Sheets	459	988
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	459	988
Total derivatives	$ 56,504	$ 321,349