CREDIT LOSSES - GROSS RECEIVABLES AND RELATED ALLOWANCES (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,629.0	$ 1,422.9
Allowance for credit losses	259.1	220.1	$ 164.8	$ 140.0
Accounts receivable and unbilled revenues, net	1,369.9	1,202.8
Total accounts receivable, net - past due greater than 90 days	$ 111.0	$ 122.8
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	95.00%	95.50%
Amount of net accounts receivable with regulatory protections	$ 742.0
Percent of net accounts receivable with regulatory protections	54.20%
Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,600.2	$ 1,373.5
Allowance for credit losses	259.1	220.1	164.7	139.9
Accounts receivable and unbilled revenues, net	1,341.1	1,153.4
Total accounts receivable, net - past due greater than 90 days	$ 111.0	$ 122.8
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	95.00%	95.50%
Wisconsin | Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,056.9	$ 899.8
Allowance for credit losses	129.5	102.1	67.7	59.9
Accounts receivable and unbilled revenues, net	927.4	797.7
Total accounts receivable, net - past due greater than 90 days	$ 68.8	$ 84.8
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	97.50%	97.60%
Illinois | Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 465.8	$ 393.9
Allowance for credit losses	122.0	111.6	93.1	75.9
Accounts receivable and unbilled revenues, net	343.8	282.3
Total accounts receivable, net - past due greater than 90 days	$ 38.4	$ 34.5
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	$ 77.5	$ 79.8
Allowance for credit losses	7.6	6.4	3.9	4.1
Accounts receivable and unbilled revenues, net	69.9	73.4
Total accounts receivable, net - past due greater than 90 days	$ 3.8	$ 3.5
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	$ 24.9	$ 45.0
Allowance for credit losses	0.0	0.0
Accounts receivable and unbilled revenues, net	24.9	45.0
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	$ 3.9	$ 4.4
Allowance for credit losses	0.0	0.0	$ 0.1	$ 0.1
Accounts receivable and unbilled revenues, net	3.9	4.4
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%