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Retirement Benefit Obligations - Additional Information (Detail) - USD ($)
$ in Millions
 6 Months Ended
Jun. 30, 2023
Jun. 30, 2022
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 2023 for pre-65 retirees is 8.00%, with an ultimate rate of 4.50% in 2030, and for post-65 retirees, the health care cost trend rate is 6.25%, with an ultimate rate of 4.50% in 2030. The health care cost trend rate used to determine benefit plan costs for 2022 for both pre-65 and post-65 retirees is 6.20%, with an ultimate rate 4.50% in 2029.  
Pension Plan [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Plan Assets, Contributions by Employer $ 1.9  
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 0.3  
Expected additional benefit payments for the remainder of 2020 $ 0.3  
Pre-65 retirees    
Defined Benefit Plan Disclosure [Line Items]    
Health care cost trend rate, assumed 8.00%  
Health care cost trend rate, ultimate rate 4.50%  
Health care cost trend rate, ultimate rate in year 2030  
Post-65 retirees    
Defined Benefit Plan Disclosure [Line Items]    
Health care cost trend rate, assumed 6.25% 6.20%
Health care cost trend rate, ultimate rate 4.50% 4.50%
Health care cost trend rate, ultimate rate in year 2030 2029