Derivative Instruments Derivative Commodity Instruments (Offsetting Asset) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Offsetting Assets [Line Items]
Derivative assets subject to enforceable master netting arrangements	$ 504	$ 0
Derivative assets not subject to enforceable master netting arrangements	5,773	462
Total derivative assets	6,277	462
Gross amounts not offset in the consolidated balance sheet	0	0
Derivative ssset, amount offset against collateral	504	0
Commodity Forward Contracts [Member]
Offsetting Assets [Line Items]
Derivative assets subject to enforceable master netting arrangements	504	0
Gross amounts of recognized assets	4,235	5,091
Gross amount offset in the consolidated balance sheet	(3,731)	(5,091)
Subject to Master Netting [Member] | Commodity Forward Contracts [Member] | Counterparty A [Member]
Offsetting Assets [Line Items]
Derivative assets subject to enforceable master netting arrangements	414	0
Gross amounts not offset in the consolidated balance sheet	0	0
Derivative ssset, amount offset against collateral	414	0
Subject to Master Netting [Member] | Commodity Forward Contracts [Member] | Counterparty B [Member]
Offsetting Assets [Line Items]
Derivative assets subject to enforceable master netting arrangements	90	0
Gross amounts not offset in the consolidated balance sheet	0	0
Derivative ssset, amount offset against collateral	$ 90	$ 0