Derivative Financial Instruments Schedule of Derivative Offset Amounts (Details) (Exchange Traded [Member], USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	$ (3,439)	$ (7,167)
Derivative Asset, Fair Value, Gross Asset	1,534	2,665
Derivative Liability Amount Not Offset	(1,905)	(4,502)
Derivative Liability, Fair Value of Collateral	12,714	14,782
Margin Deposit Net of Commodity Contract	10,809	10,280
Other Current Liabilities [Member]
Derivative [Line Items]
Derivative Liability Amount Not Offset	(3,421)	(6,737)
Other Noncurrent Assets [Member]
Derivative [Line Items]
Derivative Liability Amount Not Offset	$ 1,516	$ 2,235