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	2033
Capacity
Purchased Power Agreements (PPAs) [Abstract]
Payments for capacity	$ 56.8	$ 56.7	$ 52.4
Estimated Future Payments Under PPAs [Abstract]
2017	58.0
2018	57.0
2019	19.4
2020	11.6
2021	11.9
Thereafter	29.7
Total	$ 187.6