Derivative Financial Instruments (Schedule of Derivative Offset Amounts) (Details) - Exchange Traded [Member] - USD ($) $ in Thousands		Jun. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset		$ 8,058	$ 6,060
Derivative Liability, Fair Value, Gross Liability		(3,808)	(36,798)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral			(30,738)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		4,250
Derivative, Collateral, Right to Reclaim Cash		4,373	49,899
Amount After Offset	[1]	$ 8,623	$ 19,161

[1] Amount represents the maximum loss we would incur if all of our counterparties failed to perform on their derivative contracts.