LEASES - POWER PURCHASE COMMITMENT (Details) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($) MW	Dec. 31, 2009 USD ($)
Lessee, Lease, Description [Line Items]
Finance lease obligation	$ 48.4
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	19.7
Finance lease obligation at end of life of power purchase contract	$ 0.0