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INVESTMENT IN TRANSMISSION AFFILIATES - RELATED PARTY TRANSACTIONS (Details) - ATC - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Investment in transmission affiliates        
Charges to ATC for services and construction $ 3.3 $ 4.1 $ 7.3 $ 8.7
Charges from ATC for network transmission services 87.0 84.6 174.1 169.1
Refund from ATC related to a FERC audit $ 0.0 $ 22.0 $ 0.0 $ 22.0