LEASES - OBLIGATIONS UNDER FINANCE LEASE (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) MW	Dec. 31, 2009 USD ($)
Leases [Abstract]
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	22.1
Finance lease obligation at end of life of power purchase contract	$ 0.0