Variable Interest Entities (Details) (USD $)	1 Months Ended	3 Months Ended								12 Months Ended						12 Months Ended
	Jan. 31, 1999	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011 mW	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2002	Dec. 31, 2011 Ecova [Member]	Jan. 31, 2012 Ecova [Member]	Dec. 31, 2011 SEEL [Member]	Dec. 31, 2011 Third Party [Member]
Evaluated power capacity of Lancaster Plant, megawatts										270
Minimum estimated life of plant, in years										15
Maximum estimated life of plant, in years										25
Future contractual obligation		$ 341,000,000								$ 341,000,000
Amounts borrowed from a funding trust	145,000,000														25,000,000
Cash received from monetization of the contract	143,400,000
Maturity date of loan from funding trust										Jan. 01, 2015
Amount of cash received fully amortized by 2002													143,400,000
Percentage of partnership funded														49.00%			51.00%
Operating revenues		438,927,000	343,710,000	360,557,000	476,586,000	374,420,000	367,172,000	360,733,000	456,415,000	1,619,780,000	1,558,740,000	1,512,565,000				3,200,000
Operating expenses																1,900,000
Noncontrolling interest		$ 1,368,000	$ 935,000	$ 527,000	$ 485,000	$ 521,000	$ 988,000	$ 507,000	$ 507,000	$ 3,315,000	$ 2,523,000	$ 1,577,000				$ 700,000