Derivative Instruments Derivative Commodity Instruments (Offsetting Liability) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Offsetting Liabilities [Line Items]
Derivative liabilities not subject to enforceable master netting arrangements	$ 5,782	$ 10,868
Gross amounts not offset in the consolidated balance sheet	1,889	5,803
Total derivative liabilities	7,671	16,671
Commodity Forward Contracts [Member]
Offsetting Liabilities [Line Items]
Gross amounts not offset in the consolidated balance sheet	0	0
Gross amounts of recognized liabilities	4,224	10,894
Total derivative liabilities	1,889	5,803
Gross amounts offset in the consolidated balance sheet	(2,335)	(5,091)
Derivative liability, net amount offset against collateral	1,889	5,803
Counterparty A [Member] | Commodity Forward Contracts [Member]
Offsetting Liabilities [Line Items]
Gross amounts not offset in the consolidated balance sheet	0	0
Total derivative liabilities	1,889	5,564
Derivative liability, net amount offset against collateral	1,889	5,564
Counterparty B [Member] | Commodity Forward Contracts [Member]
Offsetting Liabilities [Line Items]
Gross amounts not offset in the consolidated balance sheet	0	0
Total derivative liabilities	0	239
Derivative liability, net amount offset against collateral	$ 0	$ 239