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Retirement Benefit Plans - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2025
Defined Benefit Plan Disclosure [Line Items]        
Change in Discount Rate   0.25%    
Increase or decrease of Net Periodic Benefit Cost (NPBC) due to change in the discount rate $ 450,800      
Defined benefit plan, assumed health care cost trend rate, description   The health care cost trend rate used to determine benefit obligations at December 31, 2024 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 obligations at December 31, 2023 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. The health care cost trend rate used to determine benefit obligations at December 31, 2022 for pre-65 retirees was 8.00%, with an ultimate rate of 4.50% in 2030, and for post-65 retirees, the health care cost trend rate was 6.25%, with an ultimate rate of 4.50% in 2030. The health care cost trend rate used to determine benefit obligations at December 31, 2022 for pre-65 retirees was 8.00%, with an ultimate rate of 4.50% in 2030, and for post-65 retirees, the health care cost trend rate was 6.25%, with an ultimate rate of 4.50% in 2030.  
Regulatory assets $ 112,000,000 $ 109,600,000    
Health Care Cost Trend Rate, Description The health care cost trend rate used to determine plan costs for 2024 for pre-65 retirees is 8.00%, with an ultimate rate of 4.50% in 2033, and for post-65 retirees, the health care cost trend rate is 6.00%, with an ultimate rate of 4.50% in 2033. The health care cost trend rate used to determine plan costs for 2023 for pre-65 retirees was 8.00%, with an ultimate rate of 4.50% in 2030, and for post-65 retirees, the health care cost trend rate was 6.25%, with an ultimate rate of 4.50% in 2030.The health care cost trend rate used to determine plan costs for 2022 for both pre-65 retirees and post-65 retirees was 6.20%, with an ultimate rate of 4.50% in 2029.   The health care cost trend rate used to determine plan costs for 2022 for both pre-65 retirees and post-65 retirees was 6.20%, with an ultimate rate of 4.50% in 2029.  
Pension Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Accumulated Benefit Obligation $ 141,600,000 146,300,000    
Defined Benefit Plan, Target Plan Asset Allocations 4.00%      
Defined Benefit Plan, Plan Assets, Contributions by Employer $ 3,758,000 3,850,000 $ 3,800,000  
Pension Plan [Member] | Equity Funds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Target Plan Asset Allocations 54.00%      
Pension Plan [Member] | Fixed Income Securities        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Target Plan Asset Allocations 42.00%      
Other Postretirement Benefit Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Plan Assets, Contributions by Employer $ 2,552,000 2,790,000 12,153,000  
Other Postretirement Benefit Plans, Defined Benefit | Equity Funds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Target Plan Asset Allocations 55.00%      
Other Postretirement Benefit Plans, Defined Benefit | Fixed Income Securities        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Target Plan Asset Allocations 45.00%      
Supplemental Employee Retirement Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Accumulated Benefit Obligation $ 12,400,000 14,400,000    
Defined Benefit Plan, Plan Assets, Contributions by Employer 679,000 665,000 637,000  
Fair Value Of Plan Assets        
Defined Benefit Plan Disclosure [Line Items]        
Pension expense 1,800,000 2,800,000 2,400,000  
Defined Benefit Obligations        
Defined Benefit Plan Disclosure [Line Items]        
Regulatory assets 14,400,000 29,800,000    
Four Zero One K Plan        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Plan Assets, Contributions by Employer $ 4,500,000 $ 4,000,000 $ 3,500,000  
Benefit Plan Costs        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Expected Long-term Return on Assets 7.50% 7.50% 7.50%  
Pre-65 retirees        
Defined Benefit Plan Disclosure [Line Items]        
Health care cost trend rate, ultimate rate 4.50% 4.50% 4.50%  
Determine plan costs ultimate rate   4.50% 4.50%  
Health care cost trend rate, assumed 8.00% 8.50% 8.00%  
Health care cost trend rate determine plan costs   8.00% 6.20%  
Health care cost trend rate, ultimate rate in year 2033 2034 2030  
Reaches Ultimate Trend Rate Year   2030    
Post-65 retirees        
Defined Benefit Plan Disclosure [Line Items]        
Health care cost trend rate, ultimate rate 4.50% 4.50% 4.50%  
Determine plan costs ultimate rate   4.50%    
Health care cost trend rate, assumed 6.00% 7.50% 6.25%  
Health care cost trend rate determine plan costs   6.25%    
Health care cost trend rate, ultimate rate in year 2033 2034 2030  
Reaches Ultimate Trend Rate Year   2030 2029  
Scenario Forecast | Pension Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Target Plan Asset Allocations       4.00%
Scenario Forecast | Pension Plan [Member] | Equity Funds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Target Plan Asset Allocations       54.00%
Scenario Forecast | Pension Plan [Member] | Fixed Income Securities        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Target Plan Asset Allocations       42.00%
Scenario Forecast | Other Postretirement Benefit Plans, Defined Benefit | Equity Funds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Target Plan Asset Allocations       55.00%
Scenario Forecast | Other Postretirement Benefit Plans, Defined Benefit | Fixed Income Securities        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Target Plan Asset Allocations       45.00%