v3.26.1
RETIREMENT BENEFITS AND ASSETS HELD IN TRUST Schedule of Net Defined Benefit Cost Components (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 3 $ 2 $ 5 $ 4
Interest cost 1 2 3 4
Expected return on plan assets (3) (2) (5) (4)
Net cost 1 2 3 4
Other Postretirement Benefits Plan        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 2 2 4 4
Interest cost 2 2 3 3
Expected return on plan assets (3) (2) (5) (4)
Amortization of unrecognized gain (1) (2) (2) (3)
Net cost $ 0 $ 0 $ 0 $ 0