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Fair value measurements (Details 3) - USD ($) $ in Thousands | Sep. 30, 2016 | Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015
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Fair value measurements [Line Items] |  |  |  |  |  |  |
Nonrecurring fair value measurement |  |  |  |  | $ 10,800 |  | $ 1,100
Concentration risks, percentage [Abstract] |  |  |  |  |  |  |
Percentage in fixed-income and other investments | 65.00% |  | 63.00% |  | 65.00% |  |
Percentage investment in common stock of large-cap companies | 18.00% |  | 19.00% |  | 18.00% |  |
Percentage investment in common stock of mid-cap companies | 9.00% |  | 9.00% |  | 9.00% |  |
Percentage investment in common stock of small-cap companies | 6.00% |  | 7.00% |  | 6.00% |  |
Percentage Investment in target date investments | 1.00% |  | 1.00% |  | 1.00% |  |
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 | $ 85,010 |  | $ 79,286 |  | $ 77,861 |  |
Fair value, measurements, recurring [Member] | Money market funds [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 2,284 |  | 1,420 |  | 1,219 |  |
Fair value, measurements, recurring [Member] | insurance contract [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 72,818 | [1] | 67,459 | [2] | 66,464 | [3] |
Fair value, measurements, recurring [Member] | Mortgage backed securities [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 9,908 |  | 9,098 |  | 7,854 |  |
Fair value, measurements, recurring [Member] | US Treasury securities [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure |  |  | 1,309 |  | 2,324 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Money market funds [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | insurance contract [Member] |  |  |  |  |  |  |
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 [Member] | Mortgage backed securities [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | US Treasury securities [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure |  |  | 0 |  | 0 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 85,010 |  | 79,286 |  | 77,861 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 2,284 |  | 1,420 |  | 1,219 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | insurance contract [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 72,818 | [1] | 67,459 | [2] | 66,464 | [3] |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Mortgage backed securities [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 9,908 |  | 9,098 |  | 7,854 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | US Treasury securities [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure |  |  | 1,309 |  | 2,324 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Money market funds [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | insurance contract [Member] |  |  |  |  |  |  |
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 [Member] | Mortgage backed securities [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure | $ 0 |  | 0 |  | 0 |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | US Treasury securities [Member] |  |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |  |
Assets, fair value disclosure |  |  | $ 0 |  | $ 0 |  |
[1] | The insurance contract invests approximately 65 percent in fixed-income investments, 18 percent in common stock of large-cap companies, 9 percent in common stock of mid-cap companies, 6 percent in common stock of small-cap companies, 1 percent in target date investments and 1 percent in cash equivalents.
[2] | The insurance contract invests approximately 63 percent in fixed-income investments, 19 percent in common stock of large-cap companies, 9 percent in common stock of mid-cap companies, 7 percent in common stock of small-cap companies, 1 percent in target date investments and 1 percent in cash equivalents.
[3] | The insurance contract invests approximately 65 percent in fixed-income investments, 18 percent in common stock of large-cap companies, 9 percent in common stock of mid-cap companies, 6 percent in common stock of small-cap companies, 1 percent in target date investments and 1 percent in cash equivalents.

X
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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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