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Retirement Plans - Projected Benefit Obligations and Fair Value of Plan Assets (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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U.S. |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Projected benefit obligations | $ 494,273 | $ 486,305
Fair value of plans' assets | 437,234 | 417,074
U.K Plan |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Projected benefit obligations | 267,200 | 261,953
Fair value of plans' assets | $ 303,957 | $ 296,943

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- Definition Amount of plan asset for defined benefit pension plan with projected benefit obligation in excess of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2417-114920 + Details
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- Definition Amount of projected benefit obligation for defined benefit pension plan with projected benefit obligation in excess of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2417-114920 + Details
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