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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, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Investment in Transmission Affiliates |  |  |
Charges to ATC for services and construction | $ 17.1 | $ 18.5 | $ 15.4
Charges from ATC for network transmission services | 349.3 | 357.3 | 289.2
Refund from ATC per FERC ROE order | (28.3) | 0.0 | $ 0.0
Accounts receivable for services provided to ATC | 1.5 | 2.2 |
Dividends Receivable | 39.9 | 35.2 |
Accounts payable for services received from ATC | $ 31.2 | $ 28.7 |

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