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Retirement Benefit Obligations - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2023
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	$ 672,000
Defined benefit plan, assumed health care cost trend rate, description	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, 2020 for both pre-65 and post-65 retirees is 6.60%, with an ultimate rate 4.50% in 2029.
Pension expense	$ 3,600	$ 7,200	$ 6,900
Regulatory assets	114,300	156,300
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated Benefit Obligation	138,300	185,100
Defined Benefit Plan, Plan Assets, Contributions by Employer	3,800	4,100	4,665
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer	12,153	8,903	4,156
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Accumulated Benefit Obligation	13,900	17,500
Defined Benefit Plan, Plan Assets, Contributions by Employer	637	637	654
Fair Value Of Plan Assets
Defined Benefit Plan Disclosure [Line Items]
Pension expense	2,400	6,100	6,500
Defined Benefit Obligations
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	29,100	86,400
Four Zero One K Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 3,500	$ 3,300	$ 3,000
Benefit Plan Costs
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate, ultimate rate	4.50%	4.50%	4.50%
Health care cost trend rate, assumed	6.20%	6.60%	7.00%
Health care cost trend rate, ultimate rate in year	2029	2029	2029
Defined Benefit Plan, Expected Long-term Return on Assets	7.50%	7.50%	7.40%
Pre-65 retirees
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate, ultimate rate	4.50%	4.50%	4.50%
Health care cost trend rate, assumed	8.00%	6.20%	6.60%
Health care cost trend rate, ultimate rate in year	2030	2029	2029
Post-65 retirees
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate, ultimate rate	4.50%	4.50%	4.50%
Health care cost trend rate, assumed	6.25%	6.20%	6.60%
Health care cost trend rate, ultimate rate in year	2030	2029	2029
Scenario Forecast \| Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations				56.00%
Scenario Forecast \| Pension Plan [Member] \| Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations				56.00%
Scenario Forecast \| Pension Plan [Member] \| 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 Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Plan Asset Allocations				5.00%