Derivative Financial Instruments (Schedule of Derivative Offset Amounts) (Details) - Exchange Traded [Member] - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability		$ 37,657	$ 38,936
Derivative Asset, Fair Value, Gross Asset		10,289	12,851
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	[1]	(27,368)	(26,085)
Derivative, Collateral, Right to Reclaim Cash		35,610	36,690
Amount After Offset	[2]	8,242	10,605
Other Current Liabilities [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral		$ (27,368)	(25,694)
Other Noncurrent Liabilities [Member]
Derivative [Line Items]
Derivative Liability, 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 at December 31, 2017. Net amount includes energy commodity derivative contracts classified as current liabilities of $27,368 at June 30, 2018.

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