Derivative Financial Instruments (Schedule of Derivative Offset Amounts) (Details) - USD ($) $ in Thousands		Jun. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Energy commodity derivatives deposits		$ 27,533	$ 0
Exchange Traded [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset		2,558	62,166
Derivative Liability, Fair Value, Gross Liability		(11,655)	(7,155)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral			55,011
Derivative Liability, Fair Value, Amount Not Offset Against Collateral		9,097
Derivative, Collateral, Right to Reclaim Cash			(37,328)
Energy commodity derivatives deposits		27,533
Amount After Offset	[1]	$ 18,436	$ 17,683

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