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Fair value measurements (Details 3) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017 | Sep. 30, 2017
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Concentration risks, percentage [Abstract] |  |  |
Percentage in fixed-income and other investments | 47.00% | 49.00% | 50.00%
Percentage investment in common stock of large-cap companies | 24.00% | 23.00% | 23.00%
Percentage investment in common stock of mid-cap companies | 13.00% | 14.00% | 13.00%
Percentage investment in common stock of small-cap companies | 12.00% | 11.00% | 11.00%
Percentage investment in target date investments | 3.00% | 2.00% | 2.00%
Percentage investment in cash and cash equivalents | 1.00% | 1.00% | 1.00%
Fair value, measurements, recurring [Member] |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 100,787 | $ 94,774 | $ 91,234
Fair value, measurements, recurring [Member] | Money market funds |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 9,948 | 6,965 | 6,204
Fair value, measurements, recurring [Member] | Insurance contract |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 80,358 | 77,388 | 74,991
Fair value, measurements, recurring [Member] | Mortgage backed securities |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 10,302 | 10,217 | 9,427
Fair value, measurements, recurring [Member] | US Treasury securities |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 179 | 204 | 612
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 | Money market funds |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 | Insurance contract |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 | Mortgage backed securities |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 | US Treasury securities |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 100,787 | 94,774 | 91,234
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Money market funds |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 9,948 | 6,965 | 6,204
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Insurance contract |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 80,358 | 77,388 | 74,991
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Mortgage backed securities |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 10,302 | 10,217 | 9,427
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | US Treasury securities |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 179 | 204 | 612
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 | Money market funds |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 | Insurance contract |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 | Mortgage backed securities |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | 0 | 0 | 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 | US Treasury securities |  |  |
Fair value measurements [Line Items] |  |  |
Assets, fair value disclosure | $ 0 | $ 0 | $ 0

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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=99377789&loc=d3e19207-110258 + Details
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