Commitments and Contingencies, Purchased Power Agreements (Details) (Capacity [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2010
Capacity [Member]
Purchased Power Agreements [Abstract]
Payments for capacity	$ 36.2		$ 39.7	$ 42.0
Estimated Future Payments Under PPAs [Abstract]
2013	40.7
2014	54.7
2015	58.8
2016	59.3
2017	60.5
Thereafter	130.0
Total	$ 404.0	[1]

[1] Excludes contingent energy payments for renewable energy PPAs