INVESTMENT IN TRANSMISSION AFFILIATES - RELATED PARTY TRANSACTIONS (Details) - ATC - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Investment in transmission affiliates
Charges to ATC for services and construction	$ 5.5	$ 5.7	$ 17.2	$ 18.7
Charges from ATC for network transmission services	90.3	87.8	270.7	262.7
Net refund from ATC related to FERC ROE orders	$ 8.7	$ 2.3	$ 7.4	$ 10.7