Commitments and Contingencies, Purchased Power Agreements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Long-term Contract for Purchase of Electric Power [Line Items]
Purchase Power Agreement Expiration (year)	2033
Capacity Payments
Purchased Power Agreements [Abstract]
Payments for capacity	$ 56.7	$ 52.4	$ 38.4
Estimated Future Payments Under PPAs [Abstract]
2016	56.8
2017	58.0
2018	57.0
2019	19.4
2020	11.8
Thereafter	42.6
Total	$ 245.6