CREDIT LOSSES - GROSS RECEIVABLES AND RELATED ALLOWANCES (Details) - USD ($) $ in Millions	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,493.8		$ 2,017.7
Allowance for credit losses	178.7	$ 213.8	199.3	$ 175.8	$ 200.6	$ 198.3
Accounts receivable and unbilled revenues, net	1,315.1		1,818.4
Total accounts receivable, net - past due greater than 90 days	$ 159.0		$ 106.7
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	95.70%		96.80%
Amount of net accounts receivable with regulatory protections	$ 767.6
Percent of net accounts receivable with regulatory protections	58.40%
Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,444.3		$ 1,988.0
Allowance for credit losses	178.7		199.3
Accounts receivable and unbilled revenues, net	1,265.6		1,788.7
Total accounts receivable, net - past due greater than 90 days	$ 159.0		$ 106.7
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	95.70%		96.80%
Wisconsin | Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 973.7		$ 1,199.4
Allowance for credit losses	76.4	90.9	82.0	78.0	85.7	84.0
Accounts receivable and unbilled revenues, net	897.3		1,117.4
Total accounts receivable, net - past due greater than 90 days	$ 76.6		$ 51.9
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	95.70%		97.00%
Illinois | Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 402.8		$ 624.2
Allowance for credit losses	97.0	116.5	111.0	91.0	107.0	105.5
Accounts receivable and unbilled revenues, net	305.8		513.2
Total accounts receivable, net - past due greater than 90 days	$ 78.8		$ 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	$ 67.8		$ 164.4
Allowance for credit losses	5.3	$ 6.4	6.3	$ 6.8	$ 7.9	$ 8.8
Accounts receivable and unbilled revenues, net	62.5		158.1
Total accounts receivable, net - past due greater than 90 days	$ 3.6		$ 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	$ 40.7		$ 25.4
Allowance for credit losses	0.0		0.0
Accounts receivable and unbilled revenues, net	40.7		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	$ 8.8		$ 4.3
Allowance for credit losses	0.0		0.0
Accounts receivable and unbilled revenues, net	8.8		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%