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INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENT (Details) - USD ($) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016
Changes to investment in Transmission Affiliates |  |  |  |
Investment in Transmission Affiliates, balance at beginning of period | $ 1,553.4 | $ 1,443.9 |  |
Add: Earnings (loss) from equity method investment | 32.8 | 41.9 |  |
Add: Capital contributions | 12.8 | 27.6 |  |
Less: Other | 0.1 | 0.1 |  |
Investment in Transmission Affiliates, balance at end of period | $ 1,598.9 | 1,513.3 |  |
ATC |  |  |  |
Investment in Transmission Affiliates |  |  |  |
Equity method investment, ownership interest (as a percent) | 60.00% |  |  |
Changes to investment in Transmission Affiliates |  |  |  |
Investment in Transmission Affiliates, balance at beginning of period | $ 1,515.8 | 1,443.9 |  |
Add: Earnings (loss) from equity method investment | 33.4 | 47.7 |  |
Add: Capital contributions | 12.0 | 24.1 |  |
Less: Other | 0.1 | 0.1 |  |
Investment in Transmission Affiliates, balance at end of period | 1,561.1 | 1,515.6 |  |
Dividends receivable from ATC | $ 0.0 |  | $ 39.9 | $ 35.2
ATC Holdco |  |  |  |
Investment in Transmission Affiliates |  |  |  |
Equity method investment, ownership interest (as a percent) | 75.00% |  |  |
Changes to investment in Transmission Affiliates |  |  |  |
Investment in Transmission Affiliates, balance at beginning of period | $ 37.6 | 0.0 |  |
Add: Earnings (loss) from equity method investment | (0.6) | (5.8) |  |
Add: Capital contributions | 0.8 | 3.5 |  |
Less: Other | 0.0 | 0.0 |  |
Investment in Transmission Affiliates, balance at end of period | $ 37.8 | $ (2.3) |  |

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