v3.26.1
INVESTMENT IN TRANSMISSION AFFILIATES - CHANGES TO INVESTMENTS (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Changes to investments in transmission affiliates        
Equity in earnings of transmission affiliates $ 62.6 $ 51.9 $ 122.1 $ 105.5
Add: Capital contributions     112.4 87.8
Investment in transmission affiliates, balance at end of period 2,497.8 2,284.6 2,497.8 2,284.6
Transmission Affiliates        
Changes to investments in transmission affiliates        
Investment in transmission affiliates, balance at beginning of period 2,369.5 2,149.0 2,280.4 2,108.9
Equity in earnings of transmission affiliates 62.6 51.9 122.1 105.5
Add: Capital contributions 36.6 45.5 112.4 87.8
Less: Distributions 48.1 46.3 94.3 102.1
Add: Other 0.1   0.1  
Investment in transmission affiliates, balance at end of period $ 2,420.7 2,200.1 $ 2,420.7 2,200.1
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 $ 2,346.7 2,121.6 $ 2,256.9 2,085.1
Equity in earnings of transmission affiliates 62.2 50.3 121.4 100.3
Add: Capital contributions 36.6 45.5 112.4 87.8
Less: Distributions 47.5 39.9 92.7 95.7
Add: Other 0.1   0.1  
Investment in transmission affiliates, balance at end of period $ 2,398.1 2,177.5 $ 2,398.1 2,177.5
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 $ 22.8 27.4 $ 23.5 23.8
Equity in earnings of transmission affiliates 0.4 1.6 0.7 5.2
Add: Capital contributions 0.0 0.0 0.0 0.0
Less: Distributions 0.6 6.4 1.6 6.4
Add: Other 0.0   0.0  
Investment in transmission affiliates, balance at end of period $ 22.6 $ 22.6 $ 22.6 $ 22.6