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Pension plans and other postretirement benefits (Tables)
 3 Months Ended
Mar. 31, 2025
Retirement Benefits [Abstract]  
Net Periodic Defined Benefit Cost for Defined Benefit Plans
The Company provides defined pension and other postretirement benefits (including health care and life insurance benefits) to qualified retired employees. Net periodic benefit for defined benefit plans consisted of the following:
Pension Benefits
Other Postretirement Benefits
Three Months Ended March 31,
(Dollars in millions)2025202420252024
Service cost$2 $2 $— $— 
Interest cost on projected benefit obligation27 29 1 1 
Expected return on plan assets(46)(51)— — 
Amortization of net actuarial gain(1)— (1)(1)
Net periodic benefit$(18)$(20)$— $—