Derivative Financial Instruments (Schedule of Derivative Offset Amounts) (Details) - Exchange Traded [Member] - USD ($) $ in Thousands		Dec. 31, 2017		Dec. 31, 2016
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability		$ (38,936)		$ (36,798)
Derivative Asset, Fair Value, Gross Asset		12,851		6,060
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		(26,085)	[1]	(30,738)
Derivative, Collateral, Obligation to Return Cash		(36,690)		(49,899)
Amount After Offset	[2]	10,605		$ 19,161
Other Current Liabilities [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		(25,694)
Other Noncurrent Liabilities [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		$ (391)

[1] Net amount includes energy commodity derivative contracts classified as current liabilities of $25,694 and noncurrent liabilities of $391.

[2] This represents the maximum amount of loss we would incur if our counterparties failed to perform on their derivative contracts.