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Investment in Transmission Affiliates - Transactions with ATC (Details) - ATC - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Investment in Transmission Affiliates |  |  |
Charges to ATC for services and construction | $ 21.8 | $ 17.1 | $ 18.5
Charges from ATC for network transmission services | 338.1 | 349.3 | 357.3
Refund from ATC related to a FERC audit | 22.0 | 0.0 | 0.0
Refund from ATC per FERC ROE order | 0.0 | 28.3 | 0.0
Accounts receivable for services provided to ATC | 3.4 | 1.5 |
Dividends Receivable | 0.0 | 39.9 | $ 35.2
Accounts payable for services received from ATC | $ 28.2 | $ 31.2 |

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