CREDIT LOSSES - GROSS RECEIVABLES AND RELATED ALLOWANCES (Details) - USD ($) $ in Millions	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,638.6		$ 1,832.1
Allowance for credit losses	142.2	$ 159.4	162.8	$ 166.9	$ 190.7	$ 193.5
Accounts receivable and unbilled revenues, net	1,496.4		1,669.3
Total accounts receivable, net - past due greater than 90 days	$ 106.3		$ 84.4
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	94.00%		93.20%
Amount of net accounts receivable with regulatory protections	$ 911.9
Percent of net accounts receivable with regulatory protections	60.90%
Public Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,578.0		$ 1,786.1
Allowance for credit losses	142.2		162.8
Accounts receivable and unbilled revenues, net	1,435.8		1,623.3
Total accounts receivable, net - past due greater than 90 days	$ 106.3		$ 84.4
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	94.00%		93.20%
Wisconsin | Public Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,059.7		$ 1,149.9
Allowance for credit losses	52.3	60.9	73.6	68.7	83.0	77.4
Accounts receivable and unbilled revenues, net	1,007.4		1,076.3
Total accounts receivable, net - past due greater than 90 days	$ 52.2		$ 51.8
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	94.60%		93.80%
Illinois | Public Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 460.7		$ 535.6
Allowance for credit losses	84.6	93.2	83.9	93.2	104.6	109.7
Accounts receivable and unbilled revenues, net	376.1		451.7
Total accounts receivable, net - past due greater than 90 days	$ 50.5		$ 30.1
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	$ 57.6		$ 100.6
Allowance for credit losses	5.3	$ 5.3	5.3	$ 5.0	$ 3.1	$ 6.4
Accounts receivable and unbilled revenues, net	52.3		95.3
Total accounts receivable, net - past due greater than 90 days	$ 3.6		$ 2.5
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	$ 54.6		$ 40.0
Allowance for credit losses	0.0		0.0
Accounts receivable and unbilled revenues, net	54.6		40.0
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	$ 6.0		$ 6.0
Allowance for credit losses	0.0		0.0
Accounts receivable and unbilled revenues, net	6.0		6.0
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%