CREDIT LOSSES - ROLLFORWARD OF ALLOWANCES (Details) $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($)
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at December 31, 2019	$ 140.0
Provision for credit losses	49.1
Provision for credit losses deferred for future recovery or refund	34.7
Write-offs charged against the allowance	(89.2)
Recovery of amounts previously written off	30.1
Balance at June 30, 2020	164.7
Utility Operations
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at December 31, 2019	139.9
Provision for credit losses	49.1
Provision for credit losses deferred for future recovery or refund	34.7
Write-offs charged against the allowance	(89.2)
Recovery of amounts previously written off	30.1
Balance at June 30, 2020	164.6
Wisconsin | Utility Operations
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at December 31, 2019	59.9
Provision for credit losses	26.0
Provision for credit losses deferred for future recovery or refund	9.1
Write-offs charged against the allowance	(37.7)
Recovery of amounts previously written off	20.6
Balance at June 30, 2020	77.9
Illinois | Utility Operations
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at December 31, 2019	75.9
Provision for credit losses	22.0
Provision for credit losses deferred for future recovery or refund	25.6
Write-offs charged against the allowance	(49.5)
Recovery of amounts previously written off	8.7
Balance at June 30, 2020	82.7
Other States | Utility Operations
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at December 31, 2019	4.1
Provision for credit losses	1.1
Provision for credit losses deferred for future recovery or refund	0.0
Write-offs charged against the allowance	(2.0)
Recovery of amounts previously written off	0.8
Balance at June 30, 2020	4.0
Corporate and Other
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at December 31, 2019	0.1
Provision for credit losses	0.0
Provision for credit losses deferred for future recovery or refund	0.0
Write-offs charged against the allowance	0.0
Recovery of amounts previously written off	0.0
Balance at June 30, 2020	$ 0.1