Leases - Power Purchase Commitment (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) MW	Dec. 31, 2009 USD ($)
Leases
Finance lease obligation	$ 2,840.9
Power purchase commitment
Leases
Power purchase contract period	25 years
Firm capacity from power purchase contract (in megawatts) | MW	236
Minimum energy requirements over remaining term of power purchase contract (in megawatts) | MW	0
Power purchase contract renewal period	10 years
Maximum regulatory asset for power purchase contract		$ 78.5
Regulatory asset at end of life of power purchase contract	$ 0.0
Finance lease obligation	18.4
Finance lease obligation at end of life of power purchase contract	$ 0.0