Fair value measurements (Details 3) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Concentration risks, percentage [Abstract]
Percentage in fixed-income and other investments	38.00%	51.00%	51.00%
Percentage investment in common stock of large-cap companies	23.00%	23.00%	22.00%
Percentage investment in common stock of mid-cap companies	9.00%	12.00%	12.00%
Percentage investment in common stock of small-cap companies	9.00%	10.00%	10.00%
Percentage investment in target date investments	4.00%	3.00%	4.00%
Percentage investment in cash and cash equivalents	17.00%	1.00%	1.00%
Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 112,113	$ 106,559	$ 104,960
Fair value, measurements, recurring \| Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	8,478	8,440	10,816
Fair value, measurements, recurring \| Insurance contract*
Fair value measurements [Line Items]
Assets, fair value disclosure	92,413	87,009	83,134
Fair value, measurements, recurring \| Mortgage-backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	10,045	9,881	10,830
Fair value, measurements, recurring \| U.S. Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	1,177	1,229	180
Fair value, measurements, recurring \| Fair value, inputs, level 2
Fair value measurements [Line Items]
Assets, fair value disclosure	112,113	106,559	104,960
Fair value, measurements, recurring \| Fair value, inputs, level 2 \| Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	8,478	8,440	10,816
Fair value, measurements, recurring \| Fair value, inputs, level 2 \| Insurance contract*
Fair value measurements [Line Items]
Assets, fair value disclosure	92,413	87,009	83,134
Fair value, measurements, recurring \| Fair value, inputs, level 2 \| Mortgage-backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	10,045	9,881	10,830
Fair value, measurements, recurring \| Fair value, inputs, level 2 \| U.S. Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 1,177	$ 1,229	$ 180

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

No definition available.

+ Details

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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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- 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=117815213&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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