CREDIT LOSSES - GROSS RECEIVABLES AND RELATED ALLOWANCES (Details) - USD ($) $ in Millions	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,414.4		$ 1,422.9
Allowance for credit losses	231.7	$ 259.1	220.1	$ 164.7	$ 164.8	$ 140.0
Accounts receivable and unbilled revenues, net	1,182.7		1,202.8
Total accounts receivable, net - past due greater than 90 days	$ 128.7		$ 122.8
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	91.60%		95.50%
Amount of net accounts receivable with regulatory protections	$ 615.6
Percent of net accounts receivable with regulatory protections	52.10%
Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 1,396.6		$ 1,373.5
Allowance for credit losses	231.7	259.1	220.1	164.6	164.7	139.9
Accounts receivable and unbilled revenues, net	1,164.9		1,153.4
Total accounts receivable, net - past due greater than 90 days	$ 128.7		$ 122.8
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	91.60%		95.50%
Wisconsin | Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 985.9		$ 899.8
Allowance for credit losses	114.4	129.5	102.1	77.9	67.7	59.9
Accounts receivable and unbilled revenues, net	871.5		797.7
Total accounts receivable, net - past due greater than 90 days	$ 69.2		$ 84.8
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	95.40%		97.60%
Illinois | Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 363.3		$ 393.9
Allowance for credit losses	109.0	122.0	111.6	82.7	93.1	75.9
Accounts receivable and unbilled revenues, net	254.3		282.3
Total accounts receivable, net - past due greater than 90 days	$ 51.9		$ 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	$ 47.4		$ 79.8
Allowance for credit losses	8.3	7.6	6.4	4.0	3.9	4.1
Accounts receivable and unbilled revenues, net	39.1		73.4
Total accounts receivable, net - past due greater than 90 days	$ 7.6		$ 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	$ 12.2		$ 45.0
Allowance for credit losses	0.0		0.0
Accounts receivable and unbilled revenues, net	12.2		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	$ 5.6		$ 4.4
Allowance for credit losses	0.0	$ 0.0	0.0	$ 0.1	$ 0.1	$ 0.1
Accounts receivable and unbilled revenues, net	5.6		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%