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		2039
Capacity
Purchased Power Agreements (PPAs) [Abstract]
Payments for capacity		$ 190.6	$ 230.6	$ 229.8
Estimated Future Payments Under PPAs [Abstract]
2017		165.8
2018		130.2
2019		85.0
2020		69.4
2021		79.4
Thereafter		300.3
Total		830.1
Energy
Estimated Future Payments Under PPAs [Abstract]
2017	[1]	91.8
2018	[1]	93.2
2019	[1]	98.7
2020	[1]	105.4
2021	[1]	139.8
Thereafter	[1]	522.7
Total	[1]	$ 1,051.6

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