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VARIABLE INTEREST ENTITIES - TRANSMISSION AFFILIATES (Details) - USD ($)
$ in Millions
Sep. 30, 2022
Jun. 30, 2022
Dec. 31, 2021
Sep. 30, 2021
Jun. 30, 2021
Dec. 31, 2020
ATC            
Variable interest entities            
Ownership interest (as a percent) 60.00%          
Equity investment $ 1,851.8 $ 1,813.6 $ 1,766.9 $ 1,758.4 $ 1,749.7 $ 1,733.5
ATC Holdco            
Variable interest entities            
Ownership interest (as a percent) 75.00%          
Equity investment $ 24.1 $ 23.6 $ 22.5 $ 32.9 $ 32.3 $ 30.8