Derivative Financial Instruments Derivative Financial Instruments (Schedule of Derivative Offset Amounts) (Details) - Exchange Traded [Member] - USD ($) $ in Thousands		Dec. 31, 2015		Dec. 31, 2014
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset		$ 48,367		$ 106,764
Derivative Liability, Fair Value, Gross Liability		(5,646)		(10,622)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		42,721	[1]	96,142	[2]
Derivative, Collateral, Obligation to Return Cash		(24,252)		(78,279)
Amount After Offset	[3]	18,469		17,863
Other Current Assets [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		39,243		87,151
Other Current Liabilities [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral				(5,413)
Other Noncurrent Assets [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		$ 3,478		$ 14,404

[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 assets, net, of $87,151, current liabilities of $5,413 and noncurrent assets of $14,404.

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