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
Purchased Power Agreements [Abstract]
Payments for capacity	$ 69.5		$ 72.7	$ 119.5
Estimated Future Payments Under PPAs [Abstract]
2015	82.9
2016	58.4
2017	35.0
2018	27.8
2019	19.0
Thereafter	47.6
Total	270.7
Energy
Estimated Future Payments Under PPAs [Abstract]
2015	49.5	[1]
2016	22.5	[1]
2017	4.0	[1]
2018	0	[1]
2019	0	[1]
Thereafter	0	[1]
Total	$ 76.0	[1]
Maximum [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Purchase Power Agreement Duration, Maximum	2032

[1] Excludes contingent energy payments for renewable energy PPAs