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Postretirement Health Care and Life Insurance Benefits (Tables)
 12 Months Ended
Dec. 31, 2025
Retirement Benefits [Abstract]  
Components of net periodic postretirement benefit credit
Net periodic postretirement benefit credit included the following components:
 Year Ended December 31,
 202520242023
 (Dollars in millions)
Service cost for benefits earned$0.3 $0.5 $0.5 
Interest cost on accumulated postretirement benefit obligation7.8 9.1 10.2 
Expected return on plan assets(0.3)(0.4)(0.5)
Amortization of prior service credit(40.8)(53.0)(53.8)
Net actuarial gain(8.5)(17.0)(2.6)
Net periodic postretirement benefit credit$(41.5)$(60.8)$(46.2)
Amounts recognized in accumulated other comprehensive income
The following includes pretax amounts recorded in “Accumulated other comprehensive income”:
 Year Ended December 31,
 202520242023
 (Dollars in millions)
Prior service credit arising during year$— $(6.5)$— 
Amortization: 
Prior service credit40.8 53.0 53.8 
Total recorded in “Accumulated other comprehensive income”
$40.8 $46.5 $53.8 
Reconciled amount of plan's funded status
The following table sets forth the plans’ funded status reconciled with the amounts shown in the consolidated balance sheets:
 
December 31,
 20252024
 (Dollars in millions)
Change in benefit obligation:
Accumulated postretirement benefit obligation at beginning of period$145.4 $178.0 
Service cost0.3 0.5 
Interest cost7.8 9.1 
Plan amendments— (6.5)
Benefits paid and administrative fees (net of Medicare Part D reimbursements)(15.2)(18.8)
Actuarial gain(8.1)(16.9)
Accumulated postretirement benefit obligation at end of period130.2 145.4 
Change in plan assets:
Fair value of plan assets at beginning of period11.3 14.3 
Actual return on plan assets0.8 0.5 
Employer contributions12.2 15.3 
Benefits paid and administrative fees (net of Medicare Part D reimbursements)(15.2)(18.8)
Fair value of plan assets at end of period9.1 11.3 
Funded status at end of period(121.1)(134.1)
Less: Current portion (included in “Accounts payable and accrued expenses”)11.9 13.7 
Noncurrent obligation (included in “Accrued postretirement benefit costs”)$109.2 $120.4 
Weighted-average assumptions used to determine the benefit obligations
The weighted-average assumptions used to determine the benefit obligations for the plans as of the end of each year were as follows:
December 31,
 20252024
Discount rate5.39 %5.70 %
Measurement dateDecember 31, 2025December 31, 2024
The weighted-average assumptions used to determine net periodic postretirement benefit credit for the plans during each period were as follows:
Year Ended December 31,
 202520242023
Discount rate5.70 %5.44 %5.70 %
Expected long-term return on plan assets (pretax)5.95 %5.75 %5.75 %
Measurement dateDecember 31, 2024December 31, 2023December 31, 2022
Assumed health care cost trend rate
The following presents information about the assumed health care cost trend rate:
Year Ended December 31,
 20252024
Pre-Medicare:
Health care cost trend rate assumed for next year7.00 %6.50 %
Rate to which the cost trend is assumed to decline (the ultimate trend rate)4.75 %4.75 %
Year that the rate reaches the ultimate trend rate20352032
Post-Medicare:
Health care cost trend rate assumed for next year (1)
6.75 %6.15 %
Rate to which the cost trend is assumed to decline (the ultimate trend rate)4.75 %4.75 %
Year that the rate reaches the ultimate trend rate20352032
(1)    An additional one-time increase of 3% was applied to claims in 2024 to reflect potential reductions in payments from Center for Medicare and Medicaid Services related to the passage of the Inflation Reduction Act in August 2022.