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Investment in Transmission Affiliates - Changes to Investment in ATC (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($) vote member | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Changes to investment in transmission affiliates |  |  |
Investment in ATC, balance at beginning of period | $ 1,443.9 |  |
Equity in earnings of transmission affiliates | 154.3 | $ 146.5 | $ 96.1
Capital contributions | 109.6 | 42.3 | 8.7
Distributions | 154.2 |  |
Other | 0.2 |  |
Investment in ATC, balance at end of period | $ 1,553.4 | 1,443.9 |
ATC |  |  |
Investment in Transmission Affiliates |  |  |
Equity interest in ATC | 60.00% |  |
Total number of members serving on the transmission affiliate's board of directors | member | 11 |  |
Number of representatives on the transmission affiliate's board of directors | member | 1 |  |
Number of votes that can be placed by each member on the transmission affiliate's board of directors | vote | 1 |  |
Maximum voting control of any member on the transmission affiliate's board of directors | 10.00% |  |
Changes to investment in transmission affiliates |  |  |
Investment in ATC, balance at beginning of period | $ 1,443.9 | 1,380.9 | 424.1
Equity in earnings of transmission affiliates | 166.0 | 146.5 | 96.1
Capital contributions | 60.3 | 42.3 | 8.7
Acquisition of Integrys's investment in ATC |  | (1.0) | 541.5
Equity method goodwill from the acquisition of Integrys |  | 10.4 | 395.8
Distributions | 154.2 | 135.1 | 85.1
Other | 0.2 | 0.1 | 0.2
Investment in ATC, balance at end of period | 1,515.8 | 1,443.9 | $ 1,380.9
Dividends not received |  |  |
Dividends Receivable | $ 39.9 | 35.2 |
ATC HoldCo |  |  |
Investment in Transmission Affiliates |  |  |
Equity interest in ATC | 75.00% |  |
Total number of members serving on the transmission affiliate's board of directors | member | 11 |  |
Number of representatives on the transmission affiliate's board of directors | member | 1 |  |
Number of votes that can be placed by each member on the transmission affiliate's board of directors | vote | 1 |  |
Maximum voting control of any member on the transmission affiliate's board of directors | 10.00% |  |
Changes to investment in transmission affiliates |  |  |
Investment in ATC, balance at beginning of period | $ 0.0 |  |
Equity in earnings of transmission affiliates | (11.7) |  |
Capital contributions | 49.3 |  |
Distributions | 0.0 |  |
Other | 0.0 |  |
Investment in ATC, balance at end of period | $ 37.6 | $ 0.0 |

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