CREDIT LOSSES - GROSS RECEIVABLES AND RELATED ALLOWANCES (Details) - USD ($) $ in Millions	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,330.5		$ 1,696.7
Allowance for credit losses	155.0	$ 166.9	193.5	$ 176.8	$ 178.7	$ 199.3
Accounts receivable and unbilled revenues, net	1,175.5		1,503.2
Total accounts receivable, net - past due greater than 90 days	$ 106.7		$ 98.8
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	93.10%		94.50%
Amount of net accounts receivable with regulatory protections	$ 670.1
Percent of net accounts receivable with regulatory protections	57.00%
Public Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,297.5		$ 1,654.4
Allowance for credit losses	155.0		193.5
Accounts receivable and unbilled revenues, net	1,142.5		1,460.9
Total accounts receivable, net - past due greater than 90 days	$ 106.7		$ 98.8
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	93.10%		94.50%
Wisconsin | Public Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 937.8		$ 1,078.0
Allowance for credit losses	63.9	68.7	77.4	72.0	76.4	82.0
Accounts receivable and unbilled revenues, net	873.9		1,000.6
Total accounts receivable, net - past due greater than 90 days	$ 52.5		$ 51.7
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	93.60%		93.60%
Illinois | Public Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 326.8		$ 481.5
Allowance for credit losses	86.9	93.2	109.7	100.0	97.0	111.0
Accounts receivable and unbilled revenues, net	239.9		371.8
Total accounts receivable, net - past due greater than 90 days	$ 50.2		$ 45.0
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	100.00%		100.00%
Other States | Public Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 32.9		$ 94.9
Allowance for credit losses	4.2	$ 5.0	6.4	$ 4.8	$ 5.3	$ 6.3
Accounts receivable and unbilled revenues, net	28.7		88.5
Total accounts receivable, net - past due greater than 90 days	$ 4.0		$ 2.1
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	0.00%		0.00%
Non-Utility Energy Infrastructure
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 26.7		$ 33.9
Allowance for credit losses	0.0		0.0
Accounts receivable and unbilled revenues, net	26.7		33.9
Total accounts receivable, net - past due greater than 90 days	$ 0.0		$ 0.0
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	0.00%		0.00%
Corporate and Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 6.3		$ 8.4
Allowance for credit losses	0.0		0.0
Accounts receivable and unbilled revenues, net	6.3		8.4
Total accounts receivable, net - past due greater than 90 days	$ 0.0		$ 0.0
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	0.00%		0.00%