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Fair value measurements (Details 3) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Concentration risks, percentage [Abstract] |  |
Percentage in fixed-income investments | 57.00% | 51.00%
Percentage investment in common stock of large-cap companies | 18.00% | 23.00%
Percentage investment in common stock of mid-cap companies | 9.00% | 12.00%
Percentage investment in common stock of small-cap companies | 9.00% | 10.00%
Percentage investment in target date investments | 5.00% | 3.00%
Percentage investment in cash and cash equivalents | 2.00% | 1.00%
Fair value, measurements, recurring |  |
Fair value measurements [Line Items] |  |
Assets, fair value disclosure | $ 120,348 | $ 106,559
Fair value, measurements, recurring | Money market funds |  |
Fair value measurements [Line Items] |  |
Assets, fair value disclosure | 8,917 | 8,440
Fair value, measurements, recurring | Insurance contract* |  |
Fair value measurements [Line Items] |  |
Assets, fair value disclosure | 100,104 | 87,009
Fair value, measurements, recurring | Mortgage-backed securities |  |
Fair value measurements [Line Items] |  |
Assets, fair value disclosure | 9,946 | 9,881
Fair value, measurements, recurring | U.S. Treasury securities |  |
Fair value measurements [Line Items] |  |
Assets, fair value disclosure | 1,381 | 1,229
Fair value, measurements, recurring | Fair value, inputs, level 2 |  |
Fair value measurements [Line Items] |  |
Assets, fair value disclosure | 120,348 | 106,559
Fair value, measurements, recurring | Fair value, inputs, level 2 | Money market funds |  |
Fair value measurements [Line Items] |  |
Assets, fair value disclosure | 8,917 | 8,440
Fair value, measurements, recurring | Fair value, inputs, level 2 | Insurance contract* |  |
Fair value measurements [Line Items] |  |
Assets, fair value disclosure | 100,104 | 87,009
Fair value, measurements, recurring | Fair value, inputs, level 2 | Mortgage-backed securities |  |
Fair value measurements [Line Items] |  |
Assets, fair value disclosure | 9,946 | 9,881
Fair value, measurements, recurring | Fair value, inputs, level 2 | U.S. Treasury securities |  |
Fair value measurements [Line Items] |  |
Assets, fair value disclosure | $ 1,381 | $ 1,229

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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