Commitments and Contingencies, Purchased Power Agreements (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Long-term Contract for Purchase of Electric Power [Line Items]
Purchase Power Agreement Expiration Date		2032
Capacity
Purchased Power Agreements (PPAs) [Abstract]
Payments for capacity		$ 44.0	$ 69.5	$ 69.5
Estimated Future Payments Under PPAs [Abstract]
2017		24.3
2018		20.1
2019		11.5
2020		3.0
2021		3.0
Thereafter		16.4
Total		78.3
Energy
Estimated Future Payments Under PPAs [Abstract]
2017	[1]	4.4
2018	[1]	0.0
2019	[1]	0.0
2020	[1]	0.0
2021	[1]	0.0
Thereafter	[1]	0.0
Total	[1]	$ 4.4

[1] Excludes contingent energy payments for renewable energy PPAs