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	$ 201.1		$ 209.1
Allowance for credit losses	10.5	$ 13.3	18.3	$ 14.7	$ 10.0	$ 4.2
Accounts receivable and unbilled revenues, net	190.6		190.8
Total accounts receivable, net - past due greater than 90 days	$ 7.5		$ 12.6
Past due greater than 90 days - collection risk mitigated by regulatory mechanisms	96.00%		94.40%
Amount of net accounts receivable with regulatory protections	$ 70.2
Percent of net accounts receivable with regulatory protections	36.80%
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable and unbilled revenues	$ 0.0		$ 0.0