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Retirement Plans and Other Postretirement Benefits - Fair Value of Plan Assets (Details) - Defined Benefit Plan - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | $ 21,945 | $ 23,164 | $ 22,090 | $ 25,843
Equity security funds, United States |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | 4,367 | 8,883 |  |
Equity security funds, International |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | 4,351 | 2,214 |  |
Fixed income funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | 13,059 | 11,116 |  |
Short-term investment funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | 168 | 951 |  |
Level 1 |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | 168 | 951 |  |
Level 1 | Equity security funds, United States |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | 0 | 0 |  |
Level 1 | Equity security funds, International |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | 0 | 0 |  |
Level 1 | Fixed income funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | 0 | 0 |  |
Level 1 | Short-term investment funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | 168 | 951 |  |
Level 2 |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | 21,777 | 22,213 |  |
Level 2 | Equity security funds, United States |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | 4,367 | 8,883 |  |
Level 2 | Equity security funds, International |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | 4,351 | 2,214 |  |
Level 2 | Fixed income funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | 13,059 | 11,116 |  |
Level 2 | Short-term investment funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | 0 | 0 |  |
Level 3 |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | 0 | 0 |  |
Level 3 | Equity security funds, United States |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | 0 | 0 |  |
Level 3 | Equity security funds, International |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | 0 | 0 |  |
Level 3 | Fixed income funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | 0 | 0 |  |
Level 3 | Short-term investment funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Plan assets amount | $ 0 | $ 0 |  |

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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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -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 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 4: 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 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(02) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 7: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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