LEASES - ELM ROAD GENERATING STATION (Details) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($) contract generating_units renewal_terms
Leases
Finance lease obligation	$ 2,850.7
Elm Road Generating Station
Leases
Power purchase contract period	30 years
Regulatory asset at end of life of power purchase contract	$ 0.0
Finance lease obligation	2,208.4
Finance lease obligation at end of life of power purchase contract	$ 0.0
Minimum number of power purchase contracts that can be renewed | contract	1
Maximum number of consecutive renewal terms | renewal_terms	3
Increase to contract term if renewal selected (as percent of remaining economic life of generation unit)	80.00%
Minimum number of generation units that can be purchased | generating_units	1
Elm Road Generating Station unit 1 (ER 1)
Leases
Maximum regulatory asset for power purchase contract	$ 525.2
Elm Road Generating Station unit 2 (ER 2)
Leases
Maximum regulatory asset for power purchase contract	$ 431.6