Derivative Financial Instruments (Schedule of Derivative Offset Amounts) (Details) - USD ($) $ in Thousands		Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Energy commodity derivatives deposits		$ 23,820	$ 0
Exchange Traded [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset		597	62,166
Derivative Liability, Fair Value, Gross Liability		(13,869)	(7,155)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral			55,011
Derivative Liability, Fair Value, Amount Not Offset Against Collateral		13,272
Derivative, Collateral, Right to Reclaim Cash			(37,328)
Energy commodity derivatives deposits		23,820
Amount After Offset	[1]	$ 10,548	$ 17,683

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