CREDIT LOSSES - ROLLFORWARD OF ALLOWANCES (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	$ 231.7	$ 164.7	$ 220.1	$ 140.0
Provision for credit losses	15.9	22.2	53.6	71.3
Write-offs charged against the allowance	(46.8)	(25.4)	(89.7)	(114.6)
Recovery of amounts previously written off	10.6	12.5	33.1	42.6
Balance at End of Period	199.7	173.3	199.7	173.3
Uncollectible expense
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses deferred for future recovery or refund	(11.7)	(0.7)	(17.4)	34.0
Utility Operations
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	231.7	164.6	220.1	139.9
Provision for credit losses	15.9	22.1	53.6	71.2
Write-offs charged against the allowance	(46.8)	(25.4)	(89.7)	(114.6)
Recovery of amounts previously written off	10.6	12.5	33.1	42.6
Balance at End of Period	199.7	173.1	199.7	173.1
Utility Operations | Uncollectible expense
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses deferred for future recovery or refund	(11.7)	(0.7)	(17.4)	34.0
Wisconsin | Utility Operations
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	114.4	77.9	102.1	59.9
Provision for credit losses	10.7	14.8	33.8	40.8
Write-offs charged against the allowance	(16.2)	(14.5)	(51.2)	(52.2)
Recovery of amounts previously written off	5.7	7.8	19.8	28.4
Balance at End of Period	76.4	88.5	76.4	88.5
Wisconsin | Utility Operations | Uncollectible expense
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses deferred for future recovery or refund	(38.2)	2.5	(28.1)	11.6
Illinois | Utility Operations
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	109.0	82.7	111.6	75.9
Provision for credit losses	4.4	6.3	16.7	28.3
Write-offs charged against the allowance	(29.9)	(10.0)	(36.7)	(59.5)
Recovery of amounts previously written off	4.7	4.4	12.4	13.1
Balance at End of Period	114.7	80.2	114.7	80.2
Illinois | Utility Operations | Uncollectible expense
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses deferred for future recovery or refund	26.5	(3.2)	10.7	22.4
Other States | Utility Operations
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	8.3	4.0	6.4	4.1
Provision for credit losses	0.8	1.0	3.1	2.1
Write-offs charged against the allowance	(0.7)	(0.9)	(1.8)	(2.9)
Recovery of amounts previously written off	0.2	0.3	0.9	1.1
Balance at End of Period	8.6	4.4	8.6	4.4
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	0.1	0.0	0.1
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	0.2	0.0	0.2
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