Forward Contracts Classified as Derivatives - Change in consolidated balance sheet location and fair values of forward contracts for purchase and sale of electricity (Details 1) (Forward Electricity Contracts, USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Forward Electricity Contracts
Derivatives, Fair Value [Line Items]
Cumulative Fair Value Adjustments Included in Earnings - Beginning of Year	$ 49	$ 894
Less: Amounts Realized on Settlement of Contracts Entered into in Prior Periods	(49)	(700)
Changes in Fair Value of Contracts Entered into in Prior Periods		(33)
Cumulative Fair Value Adjustments in Earnings of Contracts Entered into in Prior Years at End of Period		161
Changes in Fair Value of Contracts Entered into in Current Period	40	(30)
Cumulative Fair Value Adjustments Included in Earnings - End of Period	$ 40	$ 131