CREDIT LOSSES - GROSS RECEIVABLES AND RELATED ALLOWANCES (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,836.3	$ 1,704.0
Allowance for credit losses	200.6	198.3	$ 259.1	$ 220.1
Accounts receivable and unbilled revenues, net	1,635.7	1,505.7
Total accounts receivable, net - past due greater than 90 days	$ 102.9	$ 86.5
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	95.40%	94.80%
Amount of net accounts receivable with regulatory protections	$ 1,008.4
Percent of net accounts receivable with regulatory protections	61.60%
Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,810.7	$ 1,681.9
Allowance for credit losses	200.6	198.3
Accounts receivable and unbilled revenues, net	1,610.1	1,483.6
Total accounts receivable, net - past due greater than 90 days	$ 102.9	$ 86.5
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	95.40%	94.80%
Wisconsin | Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,072.2	$ 1,053.1
Allowance for credit losses	85.7	84.0	129.5	102.1
Accounts receivable and unbilled revenues, net	986.5	969.1
Total accounts receivable, net - past due greater than 90 days	$ 55.5	$ 46.5
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	98.20%	97.60%
Illinois | Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 612.8	$ 523.1
Allowance for credit losses	107.0	105.5	122.0	111.6
Accounts receivable and unbilled revenues, net	505.8	417.6
Total accounts receivable, net - past due greater than 90 days	$ 43.7	$ 36.6
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	$ 125.7	$ 105.7
Allowance for credit losses	7.9	8.8	$ 7.6	$ 6.4
Accounts receivable and unbilled revenues, net	117.8	96.9
Total accounts receivable, net - past due greater than 90 days	$ 3.7	$ 3.4
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	$ 17.1	$ 17.0
Allowance for credit losses	0.0	0.0
Accounts receivable and unbilled revenues, net	17.1	17.0
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	$ 8.5	$ 5.1
Allowance for credit losses	0.0	0.0
Accounts receivable and unbilled revenues, net	8.5	5.1
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%