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INVESTMENT IN TRANSMISSION AFFILIATES - RELATED PARTY TRANSACTIONS BALANCE SHEET INFORMATION (Details) - ATC - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017 | Dec. 31, 2016
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Investment in transmission affiliates |  |  |
Accounts receivable for services provided to ATC | $ 2.9 | $ 1.5 |
Dividends receivable from ATC | 27.7 | 39.9 | $ 35.2
Accounts payable for services received from ATC | $ 28.1 | $ 31.2 |

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