INVESTMENT IN AMERICAN TRANSMISSION COMPANY - RELATED PARTY TRANSACTIONS (Details) - ATC - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Investment in ATC
Charges to ATC for services and construction	$ 4.4	$ 4.4	$ 12.3	$ 12.8
Charges from ATC for network transmission services	87.4	89.3	262.0	271.4
Refund from ATC per FERC ROE order	$ 0.0	$ 0.0	$ (28.3)	$ 0.0