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Fair value measurements (Details 3) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017 | Mar. 31, 2017
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Concentration risks, percentage [Abstract] |  |  |  |  |  |
Percentage in fixed-income and other investments | 49.00% |  | 49.00% |  | 51.00% |
Percentage investment in common stock of large-cap companies | 22.00% |  | 23.00% |  | 22.00% |
Percentage investment in common stock of mid-cap companies | 13.00% |  | 14.00% |  | 13.00% |
Percentage investment in common stock of small-cap companies | 11.00% |  | 11.00% |  | 11.00% |
Percentage investment in target date investments | 3.00% |  | 2.00% |  | 2.00% |
Percentage investment in cash and cash equivalents | 2.00% |  | 1.00% |  | 1.00% |
Fair value, measurements, recurring [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | $ 96,554 |  | $ 94,774 |  | $ 86,624 |
Fair value, measurements, recurring [Member] | Money market funds |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 9,085 |  | 6,965 |  | 2,551 |
Fair value, measurements, recurring [Member] | Insurance contract |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 76,941 | [1] | 77,388 | [2] | 73,775 | [3]
Fair value, measurements, recurring [Member] | Mortgage backed securities |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 10,063 |  | 10,217 |  | 9,885 |
Fair value, measurements, recurring [Member] | US Treasury securities |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 465 |  | 204 |  | 413 |
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 | [1] | 0 | [2] | 0 | [3]
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 | 96,554 |  | 94,774 |  | 86,624 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Money market funds |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 9,085 |  | 6,965 |  | 2,551 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Insurance contract |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 76,941 | [1] | 77,388 | [2] | 73,775 | [3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Mortgage backed securities |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 10,063 |  | 10,217 |  | 9,885 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | US Treasury securities |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 465 |  | 204 |  | 413 |
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 | [1] | 0 | [2] | 0 | [3]
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 |
[1] | The insurance contract invests approximately 49 percent in fixed-income investments, 22 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 11 percent in common stock of small-cap companies, 3 percent in target date investments and 2 percent in cash equivalents.
[2] | The insurance contract invests approximately 49 percent in fixed-income investments, 23 percent in common stock of large-cap companies, 14 percent in common stock of mid-cap companies, 11 percent in common stock of small-cap companies, 2 percent in target date investments and 1 percent in cash equivalents.
[3] | The insurance contract invests approximately 51 percent in fixed-income investments, 22 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 11 percent in common stock of small-cap companies, 2 percent in target date investments and 1 percent in cash equivalents.

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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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