Derivative Instruments Derivative Commodity Instruments (Offsetting Liability) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Offsetting Liabilities [Line Items]
Derivative liabilities subject to enforceable master netting arrangements	$ 290	$ 5,803
Derivative liabilities not subject to enforceable master netting arrangements	3,700	10,868
Total derivative liabilities	3,990	16,671
Gross amounts not offset in the consolidated balance sheet	0	0
Derivative liability, net amount offset against collateral	290	5,803
Commodity Forward Contracts [Member]
Offsetting Liabilities [Line Items]
Derivative liabilities subject to enforceable master netting arrangements	290	5,803
Gross amounts of recognized liabilities	4,021	10,894
Gross amounts offset in the consolidated balance sheet	(3,731)	(5,091)
Counterparty A [Member] | Subject to Master Netting [Member] | Commodity Forward Contracts [Member]
Offsetting Liabilities [Line Items]
Derivative liabilities subject to enforceable master netting arrangements	290	5,564
Gross amounts not offset in the consolidated balance sheet	0	0
Derivative liability, net amount offset against collateral	290	5,564
Counterparty B [Member] | Subject to Master Netting [Member] | Commodity Forward Contracts [Member]
Offsetting Liabilities [Line Items]
Derivative liabilities subject to enforceable master netting arrangements	0	239
Gross amounts not offset in the consolidated balance sheet	0	0
Derivative liability, net amount offset against collateral	$ 0	$ 239