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INVESTMENT IN TRANSMISSION AFFILIATES - RELATED PARTY TRANSACTIONS (Details) - ATC - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Investment in transmission affiliates        
Charges to ATC for services and construction $ 5.0 $ 4.4 $ 13.7 $ 12.3
Charges from ATC for network transmission services 84.4 87.4 253.5 262.0
Refund from ATC related to a FERC audit 0.0 0.0 22.0 0.0
Refund from ATC per FERC ROE order $ 0.0 $ 0.0 $ 0.0 $ 28.3