Leases - Power Purchase Commitment (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) MW	Dec. 31, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Finance lease obligation	$ 63.4	$ 45.9
Power Purchase Contract
Lessee, Lease, Description [Line Items]
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	12.1
Finance lease obligation at end of life of power purchase contract	$ 0.0