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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, 2018 USD ($) member vote | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Changes to investment in transmission affiliates |  |  |
Equity in earnings of transmission affiliates | $ 136.7 | $ 154.3 | $ 146.5
Capital contributions | 53.5 | 109.6 | 42.3
Transmission Affiliates |  |  |
Changes to investment in transmission affiliates |  |  |
Investment in ATC, balance at beginning of period | 1,553.4 | 1,443.9 |
Equity in earnings of transmission affiliates | 136.7 | 154.3 |
Capital contributions | 53.5 | 109.6 |
Distributions | 78.2 | 154.2 |
Other | 0.1 | 0.2 |
Investment in ATC, balance at end of period | $ 1,665.3 | 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 | $ 1,515.8 | 1,443.9 | 1,380.9
Equity in earnings of transmission affiliates | 139.6 | 166.0 | 146.5
Capital contributions | 48.2 | 60.3 | 42.3
Acquisition of Integrys's investment in ATC |  |  | (1.0)
Equity method goodwill from the acquisition of Integrys |  |  | 10.4
Distributions | 78.2 | 154.2 | 135.1
Other | 0.1 | 0.2 | 0.1
Investment in ATC, balance at end of period | 1,625.3 | 1,515.8 | 1,443.9
Dividends not received |  |  |
Dividends Receivable | $ 0.0 | 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 | $ 37.6 | 0.0 |
Equity in earnings of transmission affiliates | (2.9) | (11.7) |
Capital contributions | 5.3 | 49.3 |
Distributions | 0.0 | 0.0 |
Other | 0.0 | 0.0 |
Investment in ATC, balance at end of period | $ 40.0 | $ 37.6 | $ 0.0

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