Commitments and Contingencies Qualifying Facilities Liability (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Ending QF liability	$ 184,187,000	$ 177,322,000
Qualifying Facility Contracts [Member]
Beginning QF liability	177,322,000	165,839,000
Unrecovered amount	(6,043,000)	(1,198,000)
Interest expense	12,908,000	12,681,000
Ending QF liability	184,187,000	177,322,000
Qualifying Facility Contracts [Member] | Gross Obligation [Member]
Recorded Unconditional Purchase Obligation Payment Schedule [Abstract]
2012	67,111,000
2013	69,816,000
2014	72,354,000
2015	74,135,000
2016	75,945,000
Thereafter	909,322,000
Gross contractual obligation related to QF's	1,268,683,000
Qualifying Facility Contracts [Member] | Recoverable Amounts [Member]
Recorded Unconditional Purchase Obligation Payment Schedule [Abstract]
2012	54,904,000
2013	55,462,000
2014	56,025,000
2015	56,598,000
2016	57,188,000
Thereafter	683,404,000
Gross contractual obligation related to QF's	963,581,000
Qualifying Facility Contracts [Member] | Net Amount [Member]
Recorded Unconditional Purchase Obligation Payment Schedule [Abstract]
2012	12,207,000
2013	14,354,000
2014	16,329,000
2015	17,537,000
2016	18,757,000
Thereafter	225,918,000
Gross contractual obligation related to QF's	305,102,000
Qualifying Facility Contracts [Member] | Year 2029 [Member] | Gross Obligation [Member]
Recorded unconditional purchase obligation portion recoverable through rates	1,000,000,000
Recorded Unconditional Purchase Obligation Payment Schedule [Abstract]
Gross contractual obligation related to QF's	1,300,000,000
Qualifying Facility Contracts [Member] | Minimum [Member] | Year 2029 [Member]
Price per MWH of energy required to be purchased per QF agreement	78
Qualifying Facility Contracts [Member] | Maximum [Member] | Year 2029 [Member]
Price per MWH of energy required to be purchased per QF agreement	$ 136