CREDIT LOSSES - GROSS RECEIVABLES AND RELATED ALLOWANCES (Details) - USD ($) $ in Millions	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,420.0		$ 2,017.7
Allowance for credit losses	176.8	$ 178.7	199.3	$ 168.5	$ 175.8	$ 198.3
Accounts receivable and unbilled revenues, net	1,243.2		1,818.4
Total accounts receivable, net - past due greater than 90 days	$ 127.9		$ 106.7
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	94.20%		96.80%
Amount of net accounts receivable with regulatory protections	$ 634.7
Percent of net accounts receivable with regulatory protections	51.10%
Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,373.1		$ 1,988.0
Allowance for credit losses	176.8		199.3
Accounts receivable and unbilled revenues, net	1,196.3		1,788.7
Total accounts receivable, net - past due greater than 90 days	$ 127.9		$ 106.7
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	94.20%		96.80%
Wisconsin | Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 956.7		$ 1,199.4
Allowance for credit losses	72.0	76.4	82.0	72.4	78.0	84.0
Accounts receivable and unbilled revenues, net	884.7		1,117.4
Total accounts receivable, net - past due greater than 90 days	$ 54.0		$ 51.9
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	94.00%		97.00%
Illinois | Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 355.8		$ 624.2
Allowance for credit losses	100.0	97.0	111.0	89.7	91.0	105.5
Accounts receivable and unbilled revenues, net	255.8		513.2
Total accounts receivable, net - past due greater than 90 days	$ 69.7		$ 52.9
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	100.00%		100.00%
Other States | Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 60.6		$ 164.4
Allowance for credit losses	4.8	$ 5.3	6.3	$ 6.4	$ 6.8	$ 8.8
Accounts receivable and unbilled revenues, net	55.8		158.1
Total accounts receivable, net - past due greater than 90 days	$ 4.2		$ 1.9
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	$ 40.2		$ 25.4
Allowance for credit losses	0.0		0.0
Accounts receivable and unbilled revenues, net	40.2		25.4
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%
Corporate and Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 6.7		$ 4.3
Allowance for credit losses	0.0		0.0
Accounts receivable and unbilled revenues, net	6.7		4.3
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%