CREDIT LOSSES - ROLLFORWARD OF ALLOWANCES (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	$ 220.1	$ 140.0
Provision for credit losses	22.1	28.8
Write-offs charged against the allowance	(21.8)	(52.6)
Recovery of amounts previously written off	13.3	15.8
Balance at End of Period	259.1	164.8
Uncollectible expense
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses deferred for future recovery or refund	25.4	32.8
Utility Operations
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	220.1	139.9
Provision for credit losses	22.1	28.8
Write-offs charged against the allowance	(21.8)	(52.6)
Recovery of amounts previously written off	13.3	15.8
Balance at End of Period	259.1	164.7
Utility Operations | Uncollectible expense
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses deferred for future recovery or refund	25.4	32.8
Wisconsin | Utility Operations
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	102.1	59.9
Provision for credit losses	13.7	13.7
Write-offs charged against the allowance	(18.5)	(19.7)
Recovery of amounts previously written off	9.9	10.5
Balance at End of Period	129.5	67.7
Wisconsin | Utility Operations | Uncollectible expense
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses deferred for future recovery or refund	22.3	3.3
Illinois | Utility Operations
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	111.6	75.9
Provision for credit losses	7.1	14.4
Write-offs charged against the allowance	(2.8)	(31.6)
Recovery of amounts previously written off	3.0	4.9
Balance at End of Period	122.0	93.1
Illinois | Utility Operations | Uncollectible expense
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses deferred for future recovery or refund	3.1	29.5
Other States | Utility Operations
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	6.4	4.1
Provision for credit losses	1.3	0.7
Write-offs charged against the allowance	(0.5)	(1.3)
Recovery of amounts previously written off	0.4	0.4
Balance at End of Period	7.6	3.9
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
Corporate and Other
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	0.0	0.1
Provision for credit losses	0.0	0.0
Write-offs charged against the allowance	0.0	0.0
Recovery of amounts previously written off	0.0	0.0
Balance at End of Period	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