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PENSION AND OTHER POSTRETIREMENT PLANS - Changes in Obligations and Plan Assets (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Pension Benefits |  |  |
Change in benefit obligation: |  |  |
Benefit obligation at beginning of year | $ 28,757 | $ 23,611 |
Service cost | 940 | 952 | $ 1,106
Interest cost | 794 | 908 | 794
Benefits paid and expected expenses | (609) | (475) |
Assumption changes and other | 3,865 | 3,761 |
Benefit obligation at end of year | 33,747 | 28,757 | 23,611
Change in plan assets: |  |  |
Fair value of plan assets at beginning of year | 39,745 | 33,874 |
Actual return on plan assets | 3,764 | 6,346 |
Employer contribution to the plan | 1,160 |  |
Benefits paid and actual expenses | (609) | (475) |
Fair value of plan assets at end of year | 44,060 | 39,745 | 33,874
Funded status at end of year | 10,313 | 10,988 |
SERP Benefits |  |  |
Change in benefit obligation: |  |  |
Benefit obligation at beginning of year | 5,323 | 3,811 |
Service cost | 371 | 261 | 290
Interest cost | 149 | 147 | 127
Benefits paid and expected expenses | (112) | (112) |
Assumption changes and other | 238 | 1,216 |
Benefit obligation at end of year | 5,969 | 5,323 | $ 3,811
Change in plan assets: |  |  |
Fair value of plan assets at beginning of year | 0 |  |
Actual return on plan assets | 0 | 0 |
Employer contribution to the plan | 112 | 112 |
Benefits paid and actual expenses | (112) | (112) |
Fair value of plan assets at end of year | 0 | 0 |
Funded status at end of year | $ (5,969) | $ (5,323) |

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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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