Commitments and Contingencies, Purchased Power Agreements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Long-term Contract for Purchase of Electric Power [Line Items]
Operating Lease Purchase Power Agreement Expiration Date	2033
Capacity
Purchased Power Agreements (PPAs) [Abstract]
Payments for capacity	$ 57.6	$ 58.4	$ 56.8
Estimated Future Payments Under PPAs [Abstract]
2019	20.3
2020	12.0
2021	12.2
2022	12.4
2023	12.6
Thereafter	5.7
Total	$ 75.2