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)		2032
Capacity
Purchased Power Agreements (PPAs) [Abstract]
Payments for capacity		$ 69.5	$ 69.5	$ 72.7
Estimated Future Payments Under PPAs [Abstract]
2016		44.5
2017		24.3
2018		19.2
2019		10.3
2020		1.5
Thereafter		9.5
Total		109.3
Energy
Estimated Future Payments Under PPAs [Abstract]
2016	[1]	25.3
2017	[1]	4.4
2018	[1]	0.0
2019	[1]	0.0
2020	[1]	0.0
Thereafter	[1]	0.0
Total	[1]	$ 29.7

[1] Excludes contingent energy payments for renewable energy PPAs