LEASES - FUTURE MINIMUM LEASE PAYMENTS (Details) - USD ($) $ in Millions	Sep. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Total operating leases
Three months ended December 31, 2019	$ 0.7
2020	2.7
2021	0.7
2022	0.6
2023	0.5
2024	0.5
Thereafter	13.5
Total minimum lease payments	19.2
Less: interest	(7.8)
Present value of minimum lease payments	11.4	$ 13.0
Less: short-term lease liabilities	(2.2)
Long-term lease liabilities	9.2
Total finance leases
Three months ended December 31, 2019	100.1
2020	401.2
2021	401.8
2022	396.4
2023	392.1
2024	391.9
Thereafter	5,234.7
Total minimum lease payments	7,318.2
Less: interest	(4,467.5)
Present value of minimum lease payments	2,850.7
Less: short-term lease liabilities	(55.8)		$ (49.9)
Long-term lease liabilities	2,794.9		$ 2,807.2
Power purchase commitment
Total finance leases
Three months ended December 31, 2019	2.0
2020	8.8
2021	9.4
2022	4.2
2023	0.0
2024	0.0
Thereafter	0.0
Total minimum lease payments	24.4
Less: interest	(4.7)
Present value of minimum lease payments	19.7
Less: short-term lease liabilities	(5.9)
Long-term lease liabilities	13.8
Port Washington Generating Station
Total finance leases
Three months ended December 31, 2019	24.6
2020	98.3
2021	98.3
2022	98.3
2023	98.3
2024	98.2
Thereafter	680.7
Total minimum lease payments	1,196.7
Less: interest	(574.1)
Present value of minimum lease payments	622.6
Less: short-term lease liabilities	(24.6)
Long-term lease liabilities	598.0
Elm Road Generating Station
Total finance leases
Three months ended December 31, 2019	73.5
2020	294.1
2021	294.1
2022	293.9
2023	293.8
2024	293.7
Thereafter	4,554.0
Total minimum lease payments	6,097.1
Less: interest	(3,888.7)
Present value of minimum lease payments	2,208.4
Less: short-term lease liabilities	(25.3)
Long-term lease liabilities	$ 2,183.1