CREDIT LOSSES - GROSS RECEIVABLES AND RELATED ALLOWANCES (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,747.7	$ 1,696.7
Allowance for credit losses	190.7	193.5	$ 213.8	$ 199.3
Accounts receivable and unbilled revenues, net	1,557.0	1,503.2
Total accounts receivable, net - past due greater than 90 days	$ 115.2	$ 98.8
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	95.90%	94.50%
Amount of net accounts receivable with regulatory protections	$ 1,000.4
Percent of net accounts receivable with regulatory protections	64.30%
Public Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,708.2	$ 1,654.4
Allowance for credit losses	190.7	193.5
Accounts receivable and unbilled revenues, net	1,517.5	1,460.9
Total accounts receivable, net - past due greater than 90 days	$ 115.2	$ 98.8
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	95.90%	94.50%
Wisconsin | Public Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,100.6	$ 1,078.0
Allowance for credit losses	83.0	77.4	90.9	82.0
Accounts receivable and unbilled revenues, net	1,017.6	1,000.6
Total accounts receivable, net - past due greater than 90 days	$ 68.2	$ 51.7
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	95.00%	93.60%
Illinois | Public Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 516.7	$ 481.5
Allowance for credit losses	104.6	109.7	116.5	111.0
Accounts receivable and unbilled revenues, net	412.1	371.8
Total accounts receivable, net - past due greater than 90 days	$ 45.7	$ 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	$ 90.9	$ 94.9
Allowance for credit losses	3.1	6.4	$ 6.4	$ 6.3
Accounts receivable and unbilled revenues, net	87.8	88.5
Total accounts receivable, net - past due greater than 90 days	$ 1.3	$ 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	$ 33.0	$ 33.9
Allowance for credit losses	0.0	0.0
Accounts receivable and unbilled revenues, net	33.0	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	$ 6.5	$ 8.4
Allowance for credit losses	0.0	0.0
Accounts receivable and unbilled revenues, net	6.5	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%