INVESTMENT IN TRANSMISSION AFFILIATES - RELATED PARTY TRANSACTIONS (Details) - ATC - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Investment in transmission affiliates
Charges to ATC for services and construction	$ 9.2	$ 5.0	$ 16.5	$ 13.7
Charges from ATC for network transmission services	86.9	84.4	261.0	253.5
Refund from ATC related to a FERC audit	$ 0.0	$ 0.0	$ 0.0	$ 22.0