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]
Purchase Power Agreement Expiration Date	2034
Capacity
Purchased Power Agreements (PPAs) [Abstract]
Payments for capacity	$ 20.9	$ 25.2	$ 44.0
Estimated Future Payments Under PPAs [Abstract]
2019	12.3
2020	3.3
2021	3.2
2022	3.2
2023	3.2
Thereafter	9.8
Total	$ 35.0