Commitments and Contingencies, Purchased Power Agreements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013	Dec. 31, 2012
Capacity Payments
Purchased Power Agreements [Abstract]
Payments for capacity	$ 52.4		$ 38.4	$ 36.2
Estimated Future Payments Under PPAs [Abstract]
2015	56.6
2016	57.1
2017	58.3
2018	59.6
2019	19.5
Thereafter	36.1
Total	$ 287.2	[1]
Maximum
Long-term Contract for Purchase of Electric Power [Line Items]
Purchase Power Agreement Duration, Maximum	2033

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