CREDIT LOSSES - GROSS RECEIVABLES AND RELATED ALLOWANCES (Details) - USD ($) $ in Millions	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,303.5		$ 1,422.9
Allowance for credit losses	199.7	$ 231.7	220.1	$ 173.3	$ 164.7	$ 140.0
Accounts receivable and unbilled revenues, net	1,103.8		1,202.8
Total accounts receivable, net - past due greater than 90 days	$ 87.2		$ 122.8
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	87.70%		95.50%
Amount of net accounts receivable with regulatory protections	$ 527.5
Percent of net accounts receivable with regulatory protections	47.80%
Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,289.7		$ 1,373.5
Allowance for credit losses	199.7	231.7	220.1	173.1	164.6	139.9
Accounts receivable and unbilled revenues, net	1,090.0		1,153.4
Total accounts receivable, net - past due greater than 90 days	$ 87.2		$ 122.8
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	87.70%		95.50%
Wisconsin | Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 927.9		$ 899.8
Allowance for credit losses	76.4	114.4	102.1	88.5	77.9	59.9
Accounts receivable and unbilled revenues, net	851.5		797.7
Total accounts receivable, net - past due greater than 90 days	$ 51.9		$ 84.8
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	96.50%		97.60%
Illinois | Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 316.6		$ 393.9
Allowance for credit losses	114.7	109.0	111.6	80.2	82.7	75.9
Accounts receivable and unbilled revenues, net	201.9		282.3
Total accounts receivable, net - past due greater than 90 days	$ 26.4		$ 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	$ 45.2		$ 79.8
Allowance for credit losses	8.6	8.3	6.4	4.4	4.0	4.1
Accounts receivable and unbilled revenues, net	36.6		73.4
Total accounts receivable, net - past due greater than 90 days	$ 8.9		$ 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	$ 9.4		$ 45.0
Allowance for credit losses	0.0		0.0
Accounts receivable and unbilled revenues, net	9.4		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	$ 4.4		$ 4.4
Allowance for credit losses	0.0	$ 0.0	0.0	$ 0.2	$ 0.1	$ 0.1
Accounts receivable and unbilled revenues, net	4.4		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%