CREDIT LOSSES - GROSS RECEIVABLES AND RELATED ALLOWANCES (Details) - USD ($) $ in Millions	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,490.3		$ 1,704.0
Allowance for credit losses	168.5	$ 175.8	198.3	$ 199.7	$ 231.7	$ 220.1
Accounts receivable and unbilled revenues, net	1,321.8		1,505.7
Total accounts receivable, net - past due greater than 90 days	$ 135.0		$ 86.5
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	95.10%		94.80%
Amount of net accounts receivable with regulatory protections	$ 754.7
Percent of net accounts receivable with regulatory protections	57.10%
Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,467.0		$ 1,681.9
Allowance for credit losses	168.5		198.3
Accounts receivable and unbilled revenues, net	1,298.5		1,483.6
Total accounts receivable, net - past due greater than 90 days	$ 135.0		$ 86.5
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	95.10%		94.80%
Wisconsin | Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 970.2		$ 1,053.1
Allowance for credit losses	72.4	78.0	84.0	76.4	114.4	102.1
Accounts receivable and unbilled revenues, net	897.8		969.1
Total accounts receivable, net - past due greater than 90 days	$ 56.0		$ 46.5
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	96.00%		97.60%
Illinois | Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 429.7		$ 523.1
Allowance for credit losses	89.7	91.0	105.5	114.7	109.0	111.6
Accounts receivable and unbilled revenues, net	340.0		417.6
Total accounts receivable, net - past due greater than 90 days	$ 74.6		$ 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	$ 67.1		$ 105.7
Allowance for credit losses	6.4	$ 6.8	8.8	$ 8.6	$ 8.3	$ 6.4
Accounts receivable and unbilled revenues, net	60.7		96.9
Total accounts receivable, net - past due greater than 90 days	$ 4.4		$ 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	$ 18.5		$ 17.0
Allowance for credit losses	0.0		0.0
Accounts receivable and unbilled revenues, net	18.5		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	$ 4.8		$ 5.1
Allowance for credit losses	0.0		0.0
Accounts receivable and unbilled revenues, net	4.8		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%