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INVESTMENT IN AMERICAN TRANSMISSION COMPANY - RELATED PARTY TRANSACTIONS (Details) - ATC - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Investment in ATC        
Charges to ATC for services and construction $ 3.7 $ 4.3 $ 7.9 $ 8.4
Charges from ATC for network transmission services 87.3 91.1 174.6 182.1
Refund from ATC per FERC ROE order $ 0.0 $ 0.0 $ (28.3) $ 0.0