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Retirement Benefit Obligations - Additional Information (Detail) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | 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 | $ 629 |  |  |
Pension expense | 6,900 | $ 5,800 | $ 7,600 |
Regulatory assets | $ 164,700 | 147,800 |  |
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 | $ 189,400 | 166,500 |  |
Company's contributions | 4,665 | 6,916 | 16,628 |
Other Postretirement Benefit Plans, Defined Benefit |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Company's contributions | 4,156 | 4,000 | 4,000 |
Supplemental Employee Retirement Plans, Defined Benefit |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Accumulated Benefit Obligation | 16,700 | 13,600 |  |
Company's contributions | 654 | 610 | 401 |
Fair Value Of Plan Assets |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension expense | 6,500 | 7,300 | 7,200 |
Defined Benefit Obligations |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Regulatory assets | 103,700 | 88,900 |  |
Four Zero One K Plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Company's contributions | $ 3,000 | $ 2,800 | $ 2,700 |
Benefit Plan Costs |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Expected Long-term Return on Assets | 7.40% | 7.50% | 7.75% |
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%

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- Definition Description of basis used to determine overall expected long-term rate-of-return assumption for plan asset of defined benefit plan. Includes, but is not limited to, approach used, and extent approach was based on historical return and adjustment made to reflect expectation of future return. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iii) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + 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 -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (r) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + 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 -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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