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INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016
Changes to investments in transmission affiliates |  |  |  |  |  |
Investment in transmission affiliates, balance at beginning of period | $ 1,598.9 | $ 1,513.3 | $ 1,553.4 | $ 1,443.9 |  |
Add: Earnings (loss) from equity method investment | 28.7 | 41.8 | 61.5 | 83.7 |  |
Add: Capital contributions | 19.6 | 22.9 | 32.4 | 50.5 |  |
Less: Distributions | 50.7 | 34.0 | 50.7 | 34.0 |  |
Other | 0.1 |  |  | (0.1) |  |
Investment in transmission affiliates, balance at end of period | $ 1,596.6 | 1,544.0 | $ 1,596.6 | 1,544.0 |  |
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 | $ 1,561.1 | 1,515.6 | $ 1,515.8 | 1,443.9 |  |
Add: Earnings (loss) from equity method investment | 29.8 | 42.8 | 63.2 | 90.5 |  |
Add: Capital contributions | 18.1 | 15.1 | 30.1 | 39.2 |  |
Less: Distributions | 50.7 | 34.0 | 50.7 | 34.0 |  |
Other | 0.1 |  |  | (0.1) |  |
Investment in transmission affiliates, balance at end of period | 1,558.4 | 1,539.5 | 1,558.4 | 1,539.5 |  |
Dividends receivable from ATC | $ 24.2 |  | $ 24.2 |  | $ 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 | $ 37.8 | (2.3) | $ 37.6 | 0.0 |  |
Add: Earnings (loss) from equity method investment | (1.1) | (1.0) | (1.7) | (6.8) |  |
Add: Capital contributions | 1.5 | 7.8 | 2.3 | 11.3 |  |
Less: Distributions | 0.0 | 0.0 | 0.0 | 0.0 |  |
Other | 0.0 |  |  | 0.0 |  |
Investment in transmission affiliates, balance at end of period | $ 38.2 | $ 4.5 | $ 38.2 | $ 4.5 |  |

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