LEASES - LEASE EXPENSE AND SUPPLEMENTAL CASH FLOW INFORMATION (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Lease expense
Operating lease expense	$ 0.6	$ 0.7	$ 1.9	$ 2.1
Lease expense	93.1	94.4	280.9	283.2
Amortization of finance lease right of use assets	4.9		15.4
Interest on finance lease liabilities	$ 87.6		263.6
Other information
Operating cash flows from finance / capital leases			263.6	285.8
Operating cash flows from operating leases			1.9	2.1
Financing cash flows from finance leases			37.2	0.0
Noncash activity - right of use assets obtained in exchange for new operating lease liabilities			$ 13.0
Weighted average remaining lease term - finance lease	18 years 10 months 24 days		18 years 10 months 24 days
Weighted average remaining lease term - operating leases	23 years 8 months 12 days		23 years 8 months 12 days
Weighted average discount rate - finance lease	13.90%		13.90%
Weighted average discount rate - operating leases	4.40%		4.40%
Finance and capital leases
Lease expense
Lease expense	$ 92.5	93.7	$ 279.0	281.1
Power purchase commitment
Lease expense
Lease expense	2.1	2.0	6.2	5.8
We Power leases
Lease expense
Lease expense	$ 90.4	$ 91.7	$ 272.8	$ 275.3