Fair value measurements (Details 3) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Concentration risks, percentage [Abstract]
Percentage in fixed-income and other investments	51.00%	53.00%	49.00%
Percentage investment in common stock of large-cap companies	22.00%	21.00%	22.00%
Percentage investment in common stock of mid-cap companies	12.00%	11.00%	13.00%
Percentage investment in common stock of small-cap companies	11.00%	10.00%	11.00%
Percentage investment in target date investments	3.00%	3.00%	3.00%
Percentage investment in cash and cash equivalents	1.00%	2.00%	2.00%
Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 101,584	$ 95,148	$ 96,554
Fair value, measurements, recurring \| Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	10,540	10,799	9,085
Fair value, measurements, recurring \| Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	80,243	73,838	76,941
Fair value, measurements, recurring \| Mortgage-backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	10,622	10,332	10,063
Fair value, measurements, recurring \| U.S. Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	179	179	465
Fair value, measurements, recurring \| Fair value, inputs, level 2
Fair value measurements [Line Items]
Assets, fair value disclosure	101,584	95,148	96,554
Fair value, measurements, recurring \| Fair value, inputs, level 2 \| Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	10,540	10,799	9,085
Fair value, measurements, recurring \| Fair value, inputs, level 2 \| Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	80,243	73,838	76,941
Fair value, measurements, recurring \| Fair value, inputs, level 2 \| Mortgage-backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	10,622	10,332	10,063
Fair value, measurements, recurring \| Fair value, inputs, level 2 \| U.S. Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 179	$ 179	$ 465

Concentration Risks, Percentage

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

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- Definition

Percentage of insurance contracts in cash and cash equivalents.

+ References

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- Definition

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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