Commitments and Contingencies, Guarantees and Indemnifications (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Payment or Performance Guarantee [Member]	Dec. 31, 2012 Payment or Performance Guarantee [Member]	Mar. 31, 2013 Payment or Performance Guarantee [Member] Surety Bonds [Member]	Dec. 31, 2012 Payment or Performance Guarantee [Member] Surety Bonds [Member]	Dec. 31, 2011 Indemnification Agreement [Member] Obligations Under Acquisition of Nobles Wind Project [Member] MW
Guarantees [Abstract]
Assets held as collateral	$ 0	$ 0
Guarantee amount			69,500,000	69,500,000	30,600,000	29,600,000	20,000,000
Current exposure			$ 17,800,000	$ 17,900,000
Generating capacity (in MW)							201