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
Utility segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 623.2		$ 525.4
Allowance for credit losses	47.1	$ 71.3	59.3	$ 52.1	$ 45.7	$ 38.1
Accounts receivable and unbilled revenues, net	576.1		466.1
Total accounts receivable, net - past due greater than 90 days	$ 30.3		$ 56.3
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	97.50%		98.20%
Amount of net accounts receivable with regulatory protections	$ 217.2
Percent of net accounts receivable with regulatory protections	37.70%
Other Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 0.0		$ 0.0