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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
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items] |  |  |
Other current liabilities | $ (11,987) | $ (8,826) |
Non-current liabilities | (211,880) | (212,006) |
Accumulated other comprehensive loss for unfunded benefit obligation for pensions and other postretirement benefit plans | (14,378) | (10,259) |
Pension Plan And SERP [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Benefit obligation as of beginning of year | 742,382 | 671,629 |
Service cost | 22,392 | 19,755 | $ 21,614
Interest cost | 27,853 | 28,417 | 26,096
Actuarial (gain)/loss | 74,688 | 57,829 |
Benefits paid | (40,400) | (35,248) |
Benefit obligation as of end of year | 826,915 | 742,382 | 671,629
Fair value of plan assets as of beginning of year | 642,063 | 544,051 |
Actual return on plan assets | 96,591 | 109,942 |
Employer contributions | 22,000 | 22,000 |
Benefits paid | (38,630) | (33,930) |
Fair value of plan assets as of end of year | 722,024 | 642,063 | 544,051
Funded status | (104,891) | (100,319) |
Other current liabilities | (1,943) | (1,602) |
Non-current liabilities | (102,948) | (98,717) |
Net amount recognized | (104,891) | (100,319) |
Accumulated pension benefit obligation | 710,023 | 644,004 |
Unrecognized prior service cost | 1,902 | 2,105 |
Unrecognized net actuarial loss | 119,318 | 114,368 |
Total | 121,220 | 116,473 |
Less regulatory asset | (108,301) | (107,395) |
Accumulated other comprehensive loss for unfunded benefit obligation for pensions and other postretirement benefit plans | $ 12,919 | $ 9,078 |
Discount rate for benefit obligation | 3.25% | 3.85% |
Discount rate for annual expense | 3.85% | 4.31% |
Expected long-term return on plan assets | 5.50% | 5.90% |
Rate of compensation increase | 4.74% | 4.66% |
Other Postretirement Benefits [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Benefit obligation as of beginning of year | $ 159,296 | $ 134,053 |
Service cost | 3,902 | 3,006 | 3,188
Interest cost | 6,042 | 5,598 | 4,831
Actuarial (gain)/loss | (2,589) | 23,344 |
Benefits paid | (5,418) | (6,705) |
Benefit obligation as of end of year | 161,233 | 159,296 | 134,053
Fair value of plan assets as of beginning of year | 44,853 | 36,852 |
Actual return on plan assets | 7,320 | 8,001 |
Fair value of plan assets as of end of year | 52,173 | 44,853 | $ 36,852
Funded status | (109,060) | (114,443) |
Other current liabilities | (669) | (640) |
Non-current liabilities | (108,391) | (113,803) |
Net amount recognized | (109,060) | (114,443) |
Unrecognized prior service cost | (3,570) | (4,400) |
Unrecognized net actuarial loss | 53,737 | 63,101 |
Total | 50,167 | 58,701 |
Less regulatory asset | (48,708) | (57,520) |
Accumulated other comprehensive loss for unfunded benefit obligation for pensions and other postretirement benefit plans | $ 1,459 | $ 1,181 |
Discount rate for benefit obligation | 3.27% | 3.89% |
Discount rate for annual expense | 3.89% | 4.32% |
Expected long-term return on plan assets | 5.30% | 5.70% |
Other Postretirement Benefits [Member] | Pre-Age 65 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Medical cost trend – initial | 6.25% | 5.75% |
Medical cost trend – ultimate | 5.00% | 5.00% |
Ultimate medical cost trend year | 2026 | 2023 |
Other Postretirement Benefits [Member] | Post-Age 65 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Medical cost trend – initial | 6.25% | 6.50% |
Medical cost trend – ultimate | 5.00% | 5.00% |
Ultimate medical cost trend year | 2026 | 2026 |
Other Postretirement Benefits [Member] | Retirees [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Benefit obligation as of beginning of year | $ 72,816 |  |
Benefit obligation as of end of year | 75,876 | $ 72,816 |
Other Postretirement Benefits [Member] | Fully eligible employees [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Benefit obligation as of beginning of year | 34,545 |  |
Benefit obligation as of end of year | 32,097 | 34,545 |
Other Postretirement Benefits [Member] | Other participants [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Benefit obligation as of beginning of year | 51,935 |  |
Benefit obligation as of end of year | $ 53,260 | $ 51,935 |

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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 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://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669686-108580 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (i-k) -URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669646-108580 + 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://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669686-108580 + 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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: 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 + 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 -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2919-114920 + 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 -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + 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 -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(6) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964 + 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/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(5) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964 + Details
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