Fair value measurements (Details 3) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Concentration risks, percentage [Abstract]
Percentage in fixed-income investments	53.00%	49.00%
Percentage investment in common stock of large-cap companies	21.00%	23.00%
Percentage investment in common stock of mid-cap companies	11.00%	14.00%
Percentage investment in common stock of small-cap companies	10.00%	11.00%
Percentage investment in target date investments	3.00%	2.00%
Percentage investment in cash and cash equivalents	2.00%	1.00%
Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 95,148	$ 94,774
Fair value, measurements, recurring \| Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	10,799	6,965
Fair value, measurements, recurring \| Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	73,838	77,388
Fair value, measurements, recurring \| Mortgage-backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	10,332	10,217
Fair value, measurements, recurring \| U.S. Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	179	204
Fair value, measurements, recurring \| Fair value, inputs, level 2
Fair value measurements [Line Items]
Assets, fair value disclosure	95,148	94,774
Fair value, measurements, recurring \| Fair value, inputs, level 2 \| Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	10,799	6,965
Fair value, measurements, recurring \| Fair value, inputs, level 2 \| Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	73,838	77,388
Fair value, measurements, recurring \| Fair value, inputs, level 2 \| Mortgage-backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	10,332	10,217
Fair value, measurements, recurring \| Fair value, inputs, level 2 \| U.S. Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 179	$ 204

Concentration Risks, Percentage

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- Definition

The percentage of insurance contracts in fixed-income and other investments.

+ References

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+ Details

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- Definition

The percentage of insurance contracts in common stock of large-cap companies.

+ References

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+ Details

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- Definition

The percentage of insurance contracts in common stock of mid-cap companies.

+ References

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- Definition

The percentage of insurance contracts in common stock of small-cap companies.

+ References

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- Definition

Percentage of insurance contracts in cash and cash equivalents.

+ References

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The percentage of insurance contracts in target date investments.

+ References

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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

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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