Commitments and Contingencies, Purchased Power Agreements and Guarantees and Indemnifications (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Guarantees and Indemnifications [Abstract]
Assets held as collateral	$ 0	$ 0
Payment or Performance Guarantee [Member]
Guarantees and Indemnifications [Abstract]
Guarantee amount	68,400,000	67,500,000
Current exposure	17,900,000	18,000,000
Payment or Performance Guarantee [Member] | Surety Bonds [Member]
Guarantees and Indemnifications [Abstract]
Guarantee amount	30,000,000	31,200,000
Indemnification Agreement [Member] | Obligations Under Acquisition of Nobles Wind Project [Member]
Guarantees and Indemnifications [Abstract]
Guarantee amount	20,000,000	20,000,000
Current exposure	0	0
Generating capacity (in MW)	201
Indemnification Agreement [Member] | Obligations Under Acquisition of Generation Assets [Member]
Guarantees and Indemnifications [Abstract]
Current exposure	$ 0	$ 0
Generating capacity (in MW)	900
Independent Power Producing Entities [Member]
Purchased Power Agreements [Abstract]
Generating capacity (in MW)	3,773	3,773