Derivative Financial Instruments (Schedule of Derivative Offset Amounts) (Details) - Exchange Traded [Member] - USD ($) $ in Thousands		Sep. 30, 2016	Dec. 31, 2015
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset		$ 3,247	$ 48,367
Derivative Liability, Fair Value, Gross Liability		(15,576)	(5,646)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	[1]		42,721
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	[2]	(12,329)
Derivative, Collateral, Obligation to Return Cash			(24,252)
Derivative, Collateral, Right to Reclaim Cash		30,559
Amount After Offset	[3]	18,230	18,469
Other Current Liabilities [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral		(12,430)
Other Noncurrent Assets [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral			3,478
Derivative Liability, Fair Value, Amount Not Offset Against Collateral		$ 101
Other Current Assets [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral			$ 39,243

[1] Net amount includes energy commodity derivative contracts classified as current assets, net, of $39,243 and noncurrent assets of $3,478.

[2] Net amount includes energy commodity derivative contracts classified as current liabilities, net, of $12,430 and noncurrent assets of $101.

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