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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%

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