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Fair value measurements (Details 3) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2015 | Jun. 30, 2015
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Concentration risks, percentage [Abstract] |  |  |  |  |  |
Percentage in fixed-income investments | 49.00% |  | 52.00% |  |  |
Percentage investment in common stock of large-cap companies | 23.00% |  | 22.00% |  |  |
Percentage investment in common stock of mid-cap companies | 14.00% |  | 13.00% |  |  |
Percentage investment in common stock of small-cap companies | 11.00% |  | 10.00% |  |  |
Percentage investment in target date investments | 2.00% |  | 1.00% |  |  |
Percentage investment in cash and cash equivalents | 1.00% |  | 2.00% |  |  |
Fair value, measurements, recurring [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | $ 94,774 |  | $ 82,972 |  |  |
Fair value, measurements, recurring [Member] | Money market funds |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 6,965 |  | 1,602 |  |  |
Fair value, measurements, recurring [Member] | Insurance contract |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 77,388 | [1] | 70,921 | [2] |  |
Fair value, measurements, recurring [Member] | Mortgage-backed securities |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 10,217 |  | 10,449 |  |  |
Fair value, measurements, recurring [Member] | U.S. Treasury securities |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 204 |  |  |  |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Money market funds |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Insurance contract |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 | [1] | 0 | [2] |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Mortgage-backed securities |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | U.S. Treasury securities |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 |  |  |  |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 94,774 |  | 82,972 |  |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 6,965 |  | 1,602 |  |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance contract |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 77,388 | [1] | 70,921 | [2] |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Mortgage-backed securities |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 10,217 |  | 10,449 |  |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | U.S. Treasury securities |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 204 |  |  |  |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Money market funds |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Insurance contract |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 | [1] | 0 | [2] |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Mortgage-backed securities |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | $ 0 |  |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | U.S. Treasury securities |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | $ 0 |  |  |  |  |
Pipeline and midstream: |  |  |  |  |  |
Fair value measurements [Line Items] |  |  |  |  |  |
Nonrecurring fair value measurement |  |  |  |  | $ 10,800 | $ 1,100
[1] | 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.
[2] | The insurance contract invests approximately 52 percent in fixed-income investments, 22 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 10 percent in common stock of small-cap companies, 1 percent in target date investments and 2 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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