Leases - Power Purchase Commitment (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) MW	Dec. 31, 2018 USD ($)
Lessee, Lease, Description [Line Items]
Finance lease obligation	$ 45,900,000	$ 23,300,000
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	$ 78,500,000
Regulatory asset at end of life of power purchase contract	0
Finance lease obligation	18,400,000
Finance lease obligation at end of life of power purchase contract	$ 0