CREDIT LOSSES - ROLLFORWARD OF ALLOWANCES (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	$ 259.1	$ 164.8	$ 220.1	$ 140.0
Provision for credit losses	15.6	20.3	37.7	49.1
Write-offs charged against the allowance	(21.1)	(36.6)	(42.9)	(89.2)
Recovery of amounts previously written off	9.2	14.3	22.5	30.1
Balance at End of Period	231.7	164.7	231.7	164.7
Uncollectible expense
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses deferred for future recovery or refund	(31.1)	1.9	(5.7)	34.7
Utility Operations
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	259.1	164.7	220.1	139.9
Provision for credit losses	15.6	20.3	37.7	49.1
Write-offs charged against the allowance	(21.1)	(36.6)	(42.9)	(89.2)
Recovery of amounts previously written off	9.2	14.3	22.5	30.1
Balance at End of Period	231.7	164.6	231.7	164.6
Utility Operations | Uncollectible expense
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses deferred for future recovery or refund	(31.1)	1.9	(5.7)	34.7
Wisconsin | Utility Operations
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	129.5	67.7	102.1	59.9
Provision for credit losses	9.4	12.3	23.1	26.0
Write-offs charged against the allowance	(16.5)	(18.0)	(35.0)	(37.7)
Recovery of amounts previously written off	4.2	10.1	14.1	20.6
Balance at End of Period	114.4	77.9	114.4	77.9
Wisconsin | Utility Operations | Uncollectible expense
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses deferred for future recovery or refund	(12.2)	5.8	10.1	9.1
Illinois | Utility Operations
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	122.0	93.1	111.6	75.9
Provision for credit losses	5.2	7.6	12.3	22.0
Write-offs charged against the allowance	(4.0)	(17.9)	(6.8)	(49.5)
Recovery of amounts previously written off	4.7	3.8	7.7	8.7
Balance at End of Period	109.0	82.7	109.0	82.7
Illinois | Utility Operations | Uncollectible expense
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses deferred for future recovery or refund	(18.9)	(3.9)	(15.8)	25.6
Other States | Utility Operations
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	7.6	3.9	6.4	4.1
Provision for credit losses	1.0	0.4	2.3	1.1
Write-offs charged against the allowance	(0.6)	(0.7)	(1.1)	(2.0)
Recovery of amounts previously written off	0.3	0.4	0.7	0.8
Balance at End of Period	8.3	4.0	8.3	4.0
Other States | Utility Operations | Uncollectible expense
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses deferred for future recovery or refund	0.0	0.0	0.0	0.0
Corporate and Other
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	0.0	0.1	0.0	0.1
Provision for credit losses	0.0	0.0	0.0	0.0
Write-offs charged against the allowance	0.0	0.0	0.0	0.0
Recovery of amounts previously written off	0.0	0.0	0.0	0.0
Balance at End of Period	0.0	0.1	0.0	0.1
Corporate and Other | Uncollectible expense
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses deferred for future recovery or refund	$ 0.0	$ 0.0	$ 0.0	$ 0.0