Commitments and Contingencies, Leases (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Leased Assets [Line Items]
Operating Lease Purchase Power Agreement Expiration Date		2033
Operating Leases, Future Minimum Payments Due [Abstract]
2017		$ 56.5
2018		56.4
2019		56.4
2020		56.3
2021		56.1
Thereafter		661.5
Office Space and Other Equipment
Operating Leases [Abstract]
Total expenses under operating lease obligations		56.6	$ 54.5	$ 63.1
Operating Leases, Future Minimum Payments Due [Abstract]
2017		5.0
2018		5.7
2019		5.7
2020		5.6
2021		5.4
Thereafter		67.9
Purchased Power Agreements
Operating Leases [Abstract]
Payments for capacity for PPAs under operating lease obligations		50.6	$ 48.6	$ 57.1
Operating Leases, Future Minimum Payments Due [Abstract]
2017	[1],[2]	51.5
2018	[1],[2]	50.7
2019	[1],[2]	50.7
2020	[1],[2]	50.7
2021	[1],[2]	50.7
Thereafter	[1],[2]	$ 593.6

[1] Amounts do not include PPAs accounted for as executory contracts.

[2] PPA operating leases contractually expire through 2033.