Commitments and Contingencies, Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Leased Assets [Line Items]
Operating Lease Purchase Power Agreement Expiration Date	2033
Operating Leases, Future Minimum Payments Due [Abstract]
2018	$ 51.9
2019	51.4
2020	51.3
2021	51.3
2022	51.3
Thereafter	507.1
Office Space and Other Equipment
Operating Leases [Abstract]
Total expenses under operating lease obligations	59.0	$ 57.8	$ 56.6
Operating Leases, Future Minimum Payments Due [Abstract]
2018	5.2
2019	5.2
2020	5.1
2021	5.1
2022	5.1
Thereafter	56.3
Purchased Power Agreements
Operating Leases [Abstract]
Payments for capacity	51.1	$ 51.4	$ 50.6
Operating Leases, Future Minimum Payments Due [Abstract]
2018	46.7
2019	46.2
2020	46.2
2021	46.2
2022	46.2
Thereafter	$ 450.8