Commitments and Contingencies, Purchased Power Agreements (Details) (Capacity Payments, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011
Capacity Payments
Purchased Power Agreements [Abstract]
Payments for capacity	$ 38.4		$ 36.2	$ 39.7
Estimated Future Payments Under PPAs [Abstract]
2014	52.3
2015	56.5
2016	57.0
2017	58.2
2018	57.2
Thereafter	72.2
Total	$ 353.4	[1]

[1] (a) Excludes contingent energy payments for renewable energy PPAs.