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INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2025 | Mar. 31, 2024
Changes to investments in transmission affiliates |  |
Add: Earnings from equity method investment | $ 53.6 | $ 44.8
Add: Capital contributions | 42.3 | 12.1
Investment in transmission affiliates, balance at end of period | 2,233.9 | 2,106.8
Transmission Affiliates |  |
Changes to investments in transmission affiliates |  |
Investment in transmission affiliates, balance at beginning of period | 2,108.9 | 2,005.9
Add: Earnings from equity method investment | 53.6 | 44.8
Add: Capital contributions | 42.3 | 12.1
Less: Distributions | 55.8 | 35.7
Investment in transmission affiliates, balance at end of period | $ 2,149.0 | 2,027.1
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 | $ 2,085.1 | 1,980.8
Add: Earnings from equity method investment | 50.0 | 44.4
Add: Capital contributions | 42.3 | 12.1
Less: Distributions | 55.8 | 35.7
Investment in transmission affiliates, balance at end of period | $ 2,121.6 | 2,001.6
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 | $ 23.8 | 25.1
Add: Earnings from equity method investment | 3.6 | 0.4
Add: Capital contributions | 0.0 | 0.0
Less: Distributions | 0.0 | 0.0
Investment in transmission affiliates, balance at end of period | $ 27.4 | $ 25.5

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