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Retirement Benefit Obligations - Additional Information (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2022
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 $ 679      
Pension expense 7,200 $ 6,900 $ 5,800  
Regulatory assets $ 156,300 164,700    
Defined Benefit Plan, Expected Long-term Rate-of-Return on Assets Assumption The desired investment objective is a long-term rate of return on assets that is approximately 5 – 6% greater than the assumed rate of inflation as measured by the Consumer Price Index.      
Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Accumulated Benefit Obligation $ 185,100 189,400    
Company's contributions 4,100 4,665 6,916  
Other Postretirement Benefit Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Company's contributions 8,903 4,156 4,000  
Supplemental Employee Retirement Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Accumulated Benefit Obligation 17,500 16,700    
Company's contributions 637 654 610  
Fair Value Of Plan Assets        
Defined Benefit Plan Disclosure [Line Items]        
Pension expense 6,100 6,500 7,300  
Defined Benefit Obligations        
Defined Benefit Plan Disclosure [Line Items]        
Regulatory assets 86,400 103,700    
Four Zero One K Plan        
Defined Benefit Plan Disclosure [Line Items]        
Company's contributions $ 3,300 $ 3,000 $ 2,800  
Benefit Plan Costs        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Expected Long-term Return on Assets 7.50% 7.40% 7.50%  
Scenario Forecast | Pension Plans | Equity Funds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Target Plan Asset Allocations       56.00%
Scenario Forecast | Pension Plans | Fixed Income Securities        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Target Plan Asset Allocations       39.00%
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%
Real Estate Funds | Scenario Forecast | Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Target Plan Asset Allocations       5.00%