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Retirement Benefit Plans - Additional Information (Detail) - USD ($) | 12 Months Ended |
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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%

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- Definition Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-8 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (k)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of contribution received by defined benefit plan from employer which increases plan assets. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480482/715-20-55-17 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480482/715-20-55-18 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-6 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 Reference 6: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 Reference 7: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
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- Definition Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (l) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of increase (decrease) in net periodic benefit cost from effect of change in substantive term of existing defined benefit plan or initiation of new defined benefit plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (r) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of accumulated benefit obligation for defined benefit plan with accumulated benefit obligation in excess of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480482/715-20-55-17 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-3 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-3 + Details
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- Definition Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480482/715-20-55-17 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of cost (reversal of cost) for pension benefits. Excludes other postretirement benefits. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 48 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482785/280-10-55-48 + Details
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