CREDIT LOSSES - GROSS RECEIVABLES AND RELATED ALLOWANCES (Details) - USD ($) $ in Millions	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,409.6		$ 1,696.7
Allowance for credit losses	166.9	$ 190.7	193.5	$ 178.7	$ 213.8	$ 199.3
Accounts receivable and unbilled revenues, net	1,242.7		1,503.2
Total accounts receivable, net - past due greater than 90 days	$ 125.5		$ 98.8
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	95.10%		94.50%
Amount of net accounts receivable with regulatory protections	$ 729.6
Percent of net accounts receivable with regulatory protections	58.70%
Public Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,366.8		$ 1,654.4
Allowance for credit losses	166.9		193.5
Accounts receivable and unbilled revenues, net	1,199.9		1,460.9
Total accounts receivable, net - past due greater than 90 days	$ 125.5		$ 98.8
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	95.10%		94.50%
Wisconsin | Public Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 966.0		$ 1,078.0
Allowance for credit losses	68.7	83.0	77.4	76.4	90.9	82.0
Accounts receivable and unbilled revenues, net	897.3		1,000.6
Total accounts receivable, net - past due greater than 90 days	$ 66.9		$ 51.7
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	94.40%		93.60%
Illinois | Public Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 364.0		$ 481.5
Allowance for credit losses	93.2	104.6	109.7	97.0	116.5	111.0
Accounts receivable and unbilled revenues, net	270.8		371.8
Total accounts receivable, net - past due greater than 90 days	$ 56.2		$ 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	$ 36.8		$ 94.9
Allowance for credit losses	5.0	$ 3.1	6.4	$ 5.3	$ 6.4	$ 6.3
Accounts receivable and unbilled revenues, net	31.8		88.5
Total accounts receivable, net - past due greater than 90 days	$ 2.4		$ 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	$ 37.1		$ 33.9
Allowance for credit losses	0.0		0.0
Accounts receivable and unbilled revenues, net	37.1		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	$ 5.7		$ 8.4
Allowance for credit losses	0.0		0.0
Accounts receivable and unbilled revenues, net	5.7		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%