Fair value measurements (Details 3) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Concentration risks, percentage [Abstract]
Percentage in fixed-income and other investments	63.00%	57.00%	68.00%
Percentage investment in common stock of large-cap companies	16.00%	18.00%	15.00%
Percentage investment in common stock of mid-cap companies	8.00%	9.00%	6.00%
Percentage investment in common stock of small-cap companies	7.00%	9.00%	6.00%
Percentage investment in target date investments	5.00%	5.00%	4.00%
Percentage investment in cash and cash equivalents	1.00%	2.00%	1.00%
Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 127,610	$ 120,348	$ 115,356
Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	127,610	120,348	115,356
Money market funds \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	9,907	8,917	8,478
Money market funds \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	9,907	8,917	8,478
Insurance contracts* \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	106,371	100,104	95,687
Insurance contracts* \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	106,371	100,104	95,687
Mortgage-backed securities \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	8,679	9,946	10,022
Mortgage-backed securities \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	8,679	9,946	10,022
U.S. Treasury securities \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	2,653	1,381	1,169
U.S. Treasury securities \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 2,653	$ 1,381	$ 1,169

Concentration Risks, Percentage

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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

No definition available.

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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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