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Pension Plans and Other Postretirement Benefit Plans - Change in Benefit Obligation and Plan Assets (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Other current liabilities |  | $ (14,082) | $ (15,625) |
Non-current liabilities |  | (89,830) | (93,901) |
Accumulated other comprehensive loss for unfunded benefit obligation for pensions and other postretirement benefit plans |  | (357) | (2,058) |
Pension Plan And SERP [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Benefit obligation as of beginning of year |  | 557,709 | 799,042 |
Service cost | [1] | 14,350 | 23,877 | $ 25,306
Interest cost |  | $ 33,245 | $ 26,536 | 26,160
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration] |  | Utilities Operating Expense, Maintenance, Operations, and Other Costs and Expenses | Utilities Operating Expense, Maintenance, Operations, and Other Costs and Expenses |
Actuarial (gain)/loss |  | $ 21,373 | $ (204,775) |
Plan change |  |  | 3,302 |
Settlement |  |  | (60,206) |
Benefits paid |  | (41,432) | (30,067) |
Benefit obligation as of end of year |  | 585,245 | 557,709 | 799,042
Fair value of plan assets as of beginning of year |  | 540,703 | 750,963 |
Actual return on plan assets |  | 78,838 | (163,866) |
Employer contributions |  | 10,000 | 42,000 |
Settlement |  |  | (60,206) |
Benefits paid |  | (39,558) | (28,188) |
Fair value of plan assets as of end of year |  | 589,983 | 540,703 | 750,963
Funded status |  | 4,738 | (17,006) |
Other non-current assets |  | 32,997 | 13,382 |
Other current liabilities |  | (2,212) | (1,934) |
Non-current liabilities |  | (26,047) | (28,454) |
Net amount recognized |  | 4,738 | (17,006) |
Accumulated pension benefit obligation |  | 514,295 | 495,654 |
Unrecognized prior service cost (credit) |  | 3,717 | 4,105 |
Unrecognized net actuarial loss |  | 69,002 | 83,794 |
Total |  | 72,719 | 87,899 |
Less regulatory asset |  | (71,983) | (85,198) |
Accumulated other comprehensive loss for unfunded benefit obligation for pensions and other postretirement benefit plans |  | $ 736 | $ 2,701 |
Discount rate for benefit obligation |  | 5.86% | 6.10% |
Discount rate for annual expense |  | 6.10% | 3.39% |
Expected long-term return on plan assets |  | 8.30% | 5.80% |
Rate of compensation increase |  | 4.87% | 4.69% |
Other Postretirement Benefits [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Benefit obligation as of beginning of year |  | $ 115,635 | $ 167,598 |
Service cost | [1] | 2,394 | 4,369 | 4,114
Interest cost |  | $ 6,766 | $ 5,503 | 5,139
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration] |  | Utilities Operating Expense, Maintenance, Operations, and Other Costs and Expenses | Utilities Operating Expense, Maintenance, Operations, and Other Costs and Expenses |
Actuarial (gain)/loss |  | $ 4,799 | $ (54,120) |
Benefits paid |  | (7,210) | (7,715) |
Benefit obligation as of end of year |  | 122,384 | 115,635 | 167,598
Fair value of plan assets as of beginning of year |  | 49,472 | 59,544 |
Actual return on plan assets |  | 8,654 | (10,072) |
Fair value of plan assets as of end of year |  | 58,126 | 49,472 | $ 59,544
Funded status |  | (64,258) | (66,163) |
Other current liabilities |  | (652) | (706) |
Non-current liabilities |  | (63,606) | (65,457) |
Net amount recognized |  | (64,258) | (66,163) |
Unrecognized prior service cost (credit) |  | (1,081) | (1,911) |
Unrecognized net actuarial loss |  | 13,103 | 13,643 |
Total |  | 12,022 | 11,732 |
Less regulatory asset |  | (12,401) | (12,375) |
Accumulated other comprehensive loss for unfunded benefit obligation for pensions and other postretirement benefit plans |  | $ (379) | $ (643) |
Discount rate for benefit obligation |  | 5.83% | 6.10% |
Discount rate for annual expense |  | 6.10% | 3.40% |
Expected long-term return on plan assets |  | 7.20% | 4.70% |
Other Postretirement Benefits [Member] | Pre-Age 65 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Medical cost trend – initial |  | 6.50% | 6.25% |
Medical cost trend – ultimate |  | 5.00% | 5.00% |
Ultimate medical cost trend year |  | 2030 | 2028 |
Other Postretirement Benefits [Member] | Post-Age 65 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Medical cost trend – initial |  | 6.50% | 6.25% |
Medical cost trend – ultimate |  | 5.00% | 5.00% |
Ultimate medical cost trend year |  | 2030 | 2028 |
Other Postretirement Benefits [Member] | Retirees [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Benefit obligation as of beginning of year |  | $ 61,984 |  |
Benefit obligation as of end of year |  | 68,087 | $ 61,984 |
Other Postretirement Benefits [Member] | Fully eligible employees [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Benefit obligation as of beginning of year |  | 19,731 |  |
Benefit obligation as of end of year |  | 16,054 | 19,731 |
Other Postretirement Benefits [Member] | Other participants [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Benefit obligation as of beginning of year |  | 33,920 |  |
Benefit obligation as of end of year |  | $ 38,243 | $ 33,920 |
[1] | Total service costs in the table above are recorded to the same accounts as labor expense. Labor and benefits expense is recorded to various projects based on whether the work is a capital project or an operating expense. Approximately 40 percent of all labor and benefits is capitalized to utility property and 60 percent is expensed to utility other operating expenses.

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- Definition Accumulated comprehensive income loss pension and other postretirement benefit plans regulatory asset in net periodic benefit cost net of tax. + References No definition available. + Details
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- Definition Accumulated other comprehensive income loss pension and other postretirement benefit plans before regulatory asset net of tax. + References No definition available. + Details
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- Definition Amount, after tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -Publisher FASB -URI https://asc.fasb.org//1943274/2147482790/220-10-45-14A Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 45 -Paragraph 10A -Subparagraph (i-k) -SubTopic 10 -Topic 220 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482790/220-10-45-10A Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (j) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount, after tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -Publisher FASB -URI https://asc.fasb.org//1943274/2147482790/220-10-45-14A Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (j) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount, after tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -Publisher FASB -URI https://asc.fasb.org//1943274/2147482790/220-10-45-14A Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (j) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses. + 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 (b)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a)(4) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of asset (liability), recognized in statement of financial position, for defined benefit pension and other postretirement plans. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480535/715-20-45-2 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation 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 1 -Subparagraph (k)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Weighted average rate for present value of future retirement benefits cash flows, 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 1 -Subparagraph (k)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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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 Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (k)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 60 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480266/715-60-50-4 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a)(6) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of payment, which decreases benefit obligation of defined benefit plan, for irrevocable action relieving primary responsibility for benefit obligation and eliminating risk for obligation and assets used to effect settlement. Transaction constituting settlement includes, but is not limited to, making lump-sum cash payment to participant in exchange for their rights to receive specified benefits and purchasing nonparticipating annuity contract. Excludes decreases to benefit obligation for remeasurement due to settlement. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a)(10) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee. + 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)(02) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -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 4: 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 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b) -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 payment to participant under defined benefit plan which decreases plan assets. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 715 -SubTopic 60 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480266/715-60-50-4 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(5) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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