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Retirement Benefit Obligations - Additional Information (Detail) - USD ($)
$ in Millions
 6 Months Ended 9 Months Ended
Jun. 30, 2025
Sep. 30, 2025
Sep. 30, 2024
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, assumed health care cost trend rate, description   The health care cost trend rate used to determine benefit plan costs for 2025 for pre-65 retirees is 8.50%, with an ultimate rate of 4.50% in 2034, and for post-65 retirees, the health care cost trend rate is 7.50%, with an ultimate rate of 4.50% in 2034. The health care cost trend rate used to determine benefit plan costs for 2024 for pre-65 retirees was 8.00%, with an ultimate rate of 4.50% in 2033, and for post-65 retirees, the health care cost trend rate was 6.00%, with an ultimate rate of 4.50% in 2033.  
Pension Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Plan Assets, Contributions by Employer   $ 3.9  
Other Postretirement Benefits Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Plan Assets, Contributions by Employer   $ 0.6  
Supplemental Employee Retirement Plans, Defined Benefit      
Defined Benefit Plan Disclosure [Line Items]      
Benefit payments under SERP Plan $ 0.5    
Expected additional benefit payments for the remainder of 2020 $ 0.2    
Pre-65 retirees      
Defined Benefit Plan Disclosure [Line Items]      
Health care cost trend rate, assumed   8.50% 8.00%
Health care cost trend rate, ultimate rate   4.50% 4.50%
Health care cost trend rate, ultimate rate in year   2034 2033
Post-65 retirees      
Defined Benefit Plan Disclosure [Line Items]      
Health care cost trend rate, assumed   7.50% 6.00%
Health care cost trend rate, ultimate rate   4.50% 4.50%
Health care cost trend rate, ultimate rate in year   2034 2033