Fair value measurements (Details 3) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021	Sep. 30, 2021
Concentration risks, percentage [Abstract]
Percentage in fixed-income and other investments	65.00%	61.00%	63.00%
Percentage investment in common stock of large-cap companies	14.00%	17.00%	16.00%
Percentage investment in common stock of mid-cap companies	7.00%	8.00%	8.00%
Percentage investment in common stock of small-cap companies	6.00%	7.00%	7.00%
Percentage investment in target date investments	6.00%	5.00%	5.00%
Percentage investment in cash and cash equivalents	2.00%	2.00%	1.00%
Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 112,568	$ 130,895	$ 127,610
Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	112,568	130,895	127,610
Money market funds \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	7,309	10,190	9,907
Money market funds \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	7,309	10,190	9,907
Insurance contracts* \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	95,170	109,603	106,371
Insurance contracts* \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	95,170	109,603	106,371
Mortgage-backed securities \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	7,703	8,706	8,679
Mortgage-backed securities \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	7,703	8,706	8,679
U.S. Treasury securities \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	2,386	2,396	2,653
U.S. Treasury securities \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 2,386	$ 2,396	$ 2,653

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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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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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 https://asc.fasb.org/extlink&oid=126976982&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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