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INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016
Changes to investments in transmission affiliates |  |  |  |  |  |
Investment in transmission affiliates, balance at beginning of period | $ 1,596.6 | $ 1,544.0 | $ 1,553.4 | $ 1,443.9 |  |
Add: Earnings (loss) from equity method investment | 33.7 | 39.2 | 95.2 | 122.9 |  |
Add: Capital contributions | 11.3 | 12.8 | 43.7 | 63.3 |  |
Less: Distributions | 27.8 | 35.2 | 78.5 | 69.2 |  |
Less: Other | 0.1 |  | 0.1 | 0.1 |  |
Investment in transmission affiliates, balance at end of period | $ 1,613.7 | 1,560.8 | $ 1,613.7 | 1,560.8 |  |
ATC |  |  |  |  |  |
Investment in transmission affiliates |  |  |  |  |  |
Equity method investment, ownership interest (as a percent) | 60.00% |  | 60.00% |  |  |
Changes to investments in transmission affiliates |  |  |  |  |  |
Investment in transmission affiliates, balance at beginning of period | $ 1,558.4 | 1,539.5 | $ 1,515.8 | 1,443.9 |  |
Add: Earnings (loss) from equity method investment | 34.6 | 40.9 | 97.8 | 131.4 |  |
Add: Capital contributions | 9.1 | 12.1 | 39.2 | 51.3 |  |
Less: Distributions | 27.8 | 35.2 | 78.5 | 69.2 |  |
Less: Other | 0.1 |  | 0.1 | 0.1 |  |
Investment in transmission affiliates, balance at end of period | 1,574.2 | 1,557.3 | 1,574.2 | 1,557.3 |  |
Dividends receivable from ATC | $ 27.7 |  | $ 27.7 |  | $ 39.9 | $ 35.2
ATC Holdco |  |  |  |  |  |
Investment in transmission affiliates |  |  |  |  |  |
Equity method investment, ownership interest (as a percent) | 75.00% |  | 75.00% |  |  |
Changes to investments in transmission affiliates |  |  |  |  |  |
Investment in transmission affiliates, balance at beginning of period | $ 38.2 | 4.5 | $ 37.6 | 0.0 |  |
Add: Earnings (loss) from equity method investment | (0.9) | (1.7) | (2.6) | (8.5) |  |
Add: Capital contributions | 2.2 | 0.7 | 4.5 | 12.0 |  |
Less: Distributions | 0.0 | 0.0 | 0.0 | 0.0 |  |
Less: Other | 0.0 |  | 0.0 | 0.0 |  |
Investment in transmission affiliates, balance at end of period | $ 39.5 | $ 3.5 | $ 39.5 | $ 3.5 |  |

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