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INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2023 | Mar. 31, 2022
Changes to investments in transmission affiliates |  |
Add: Earnings from equity method investment | $ 43.8 | $ 41.7
Add: Capital contributions | 6.1 | 21.1
Transmission Affiliates |  |
Changes to investments in transmission affiliates |  |
Investment in transmission affiliates, balance at beginning of period | 1,909.2 | 1,789.4
Add: Earnings from equity method investment | 43.8 | 41.7
Add: Capital contributions | 6.1 | 21.1
Less: Distributions | 37.4 | 34.0
Investment in transmission affiliates, balance at end of period | $ 1,921.7 | 1,818.2
ATC |  |
Investment in transmission affiliates |  |
Equity method investment, ownership interest (as a percent) | 60.00% |
Changes to investments in transmission affiliates |  |
Investment in transmission affiliates, balance at beginning of period | $ 1,884.6 | 1,766.9
Add: Earnings from equity method investment | 42.9 | 41.0
Add: Capital contributions | 6.1 | 21.1
Less: Distributions | 37.4 | 34.0
Investment in transmission affiliates, balance at end of period | $ 1,896.2 | 1,795.0
ATC Holdco |  |
Investment in transmission affiliates |  |
Equity method investment, ownership interest (as a percent) | 75.00% |
Changes to investments in transmission affiliates |  |
Investment in transmission affiliates, balance at beginning of period | $ 24.6 | 22.5
Add: Earnings from equity method investment | 0.9 | 0.7
Add: Capital contributions | 0.0 | 0.0
Less: Distributions | 0.0 | 0.0
Investment in transmission affiliates, balance at end of period | $ 25.5 | $ 23.2

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