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Retirement Benefit Obligations - Additional Information (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
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 $ 113,200    
Defined benefit plan, assumed health care cost trend rate, description The health care cost trend rate used to determine benefit obligations at December 31, 2023 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 benefit obligations at December 31, 2022 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 obligations at December 31, 2021 for both pre-65 and post-65 retirees is 6.20%, with an ultimate rate 4.50% in 2029. The health care cost trend rate used to determine benefit obligations at December 31, 2022 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 obligations at December 31, 2021 for both pre-65 and post-65 retirees is 6.20%, with an ultimate rate 4.50% in 2029  
Regulatory assets $ 109,600 $ 114,300  
Health Care Cost Trend Rate, Description The health care cost trend rate used to determine 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 plan costs for 2022 for both pre-65 retirees and post-65 retirees is 6.20%, with an ultimate rate of 4.50% in 2029. The health care cost trend rate used to determine plan costs for 2021 for both pre-65 retirees and post-65 retirees is 6.60%, 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 is 6.20%, with an ultimate rate of 4.50% in 2029. The health care cost trend rate used to determine plan costs for 2021 for both pre-65 retirees and post-65 retirees is 6.60%, with an ultimate rate of 4.50% in 2029.  
Pension Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Accumulated Benefit Obligation $ 146,300 $ 138,300  
Defined Benefit Plan, Plan Assets, Contributions by Employer 3,850 3,800 $ 4,100
Other Postretirement Benefit Plans, Defined Benefit      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Plan Assets, Contributions by Employer 2,790 12,153 8,903
Supplemental Employee Retirement Plans, Defined Benefit      
Defined Benefit Plan Disclosure [Line Items]      
Accumulated Benefit Obligation 14,400 13,900  
Defined Benefit Plan, Plan Assets, Contributions by Employer 665 637 637
Fair Value Of Plan Assets      
Defined Benefit Plan Disclosure [Line Items]      
Pension expense 2,800 2,400 6,100
Defined Benefit Obligations      
Defined Benefit Plan Disclosure [Line Items]      
Regulatory assets 29,800 29,100  
Four Zero One K Plan      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Plan Assets, Contributions by Employer $ 4,000 $ 3,500 $ 3,300
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% 8.00% 6.20%
Health care cost trend rate determine plan costs 8.00% 6.20%  
Health care cost trend rate, ultimate rate in year 2033 2030 2029
Reaches Ultimate Trend Rate Year 2030   2029
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% 6.25% 6.20%
Health care cost trend rate determine plan costs 6.25%    
Health care cost trend rate, ultimate rate in year 2033 2030 2029
Reaches Ultimate Trend Rate Year 2030 2029  
Pre-65 retirees and Post -65 retirees      
Defined Benefit Plan Disclosure [Line Items]      
Determine plan costs ultimate rate     4.50%
Health care cost trend rate determine plan costs     6.60%