LEASES - PORT WASHINGTON GENERATING STATION (Details) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($) contract generating_units renewal_terms MW
Leases
Finance lease obligation	$ 2,850.7
Port Washington Generating Station
Leases
Number of generation units at the Port Washington Generating Station | generating_units	2
Capacity of generation unit (in megawatts) | MW	545
Power purchase contract period	25 years
Regulatory asset at end of life of power purchase contract	$ 0.0
Finance lease obligation	622.6
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
Port Washington Generating Station unit 1 (PWGS 1)
Leases
Maximum regulatory asset for power purchase contract	$ 129.4
Port Washington Generating Station unit 2 (PWGS 2)
Leases
Maximum regulatory asset for power purchase contract	$ 126.3