Derivative Commodity Instruments Derivative Commodity Instruments (Offsetting Liability) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Offsetting Liabilities [Line Items]
Derivative liabilities subject to enforceable master netting arrangements	$ 5,803	$ 1,490
Derivative liabilities not subject to enforceable master netting arrangements	10,868	7,917
Total derivative liabilities	16,671	9,407
Gross amounts not offset in the consolidated balance sheet	0	0
Derivative liability, net amount offset against collateral	5,803	1,490
Commodity Forward Contracts [Member]
Offsetting Liabilities [Line Items]
Derivative liabilities subject to enforceable master netting arrangements	5,803	1,490
Gross amounts of recognized liabilities	10,894	2,191
Gross amounts offset in the consolidated balance sheet	(5,091)	(701)
Subject to Master Netting [Member] | Commodity Forward Contracts [Member] | Counterparty A [Member]
Offsetting Liabilities [Line Items]
Derivative liabilities subject to enforceable master netting arrangements	5,564	1,490
Gross amounts not offset in the consolidated balance sheet	0	0
Derivative liability, net amount offset against collateral	5,564	1,490
Subject to Master Netting [Member] | Commodity Forward Contracts [Member] | Counterparty B [Member]
Offsetting Liabilities [Line Items]
Derivative liabilities subject to enforceable master netting arrangements	239	0
Gross amounts not offset in the consolidated balance sheet	0	0
Derivative liability, net amount offset against collateral	$ 239	$ 0