Commitments and Contingencies, Purchased Power Agreements (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Long-term Contract for Purchase of Electric Power [Line Items]
Purchase Power Agreement Expiration (year)		2039
Capacity
Purchased Power Agreements (PPAs) [Abstract]
Payments for capacity		$ 230.6	$ 229.8	$ 217.0
Estimated Future Payments Under PPAs [Abstract]
2016		191.0
2017		165.8
2018		129.1
2019		83.6
2020		68.1
Thereafter		362.5
Total		1,000.1
Energy
Estimated Future Payments Under PPAs [Abstract]
2016	[1]	106.9
2017	[1]	91.8
2018	[1]	93.2
2019	[1]	98.7
2020	[1]	105.4
Thereafter	[1]	662.5
Total	[1]	$ 1,158.5

[1] Excludes contingent energy payments for renewable energy PPAs.