Summary of Significant Accounting Policies - Changes in Level 3 forward energy contract derivative asset and liability fair valuations (Details 5) (Forward Energy Contracts, Level 3, USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Forward Energy Contracts | Level 3
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Forward Energy Contracts - Fair Values Beginning of Period	$ (17,782)
Transfers into Level 3 from Level 2		(15,884)
Less: Amounts Reversed on Settlement of Contracts Entered into in Prior Periods	3,776	2,861
Changes in Fair Value of Contracts Entered into in Prior Periods	1,851	(5,046)
Cumulative Fair Value Adjustments of Contracts Entered into in Prior Years at End of Period	(12,155)	(18,069)
Net Increase in Value of Open Contracts Entered into in Current Period	41
Forward Energy Contracts - Net Derivative Liability Fair Values End of Period	$ (12,114)	$ (18,069)