Fair value measurements (Details 3) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021
Concentration risks, percentage [Abstract]
Percentage in fixed-income and other investments	61.00%	61.00%	54.00%
Percentage investment in common stock of large-cap companies	17.00%	17.00%	19.00%
Percentage investment in common stock of mid-cap companies	8.00%	8.00%	10.00%
Percentage investment in common stock of small-cap companies	6.00%	7.00%	9.00%
Percentage investment in target date investments	6.00%	5.00%	6.00%
Percentage investment in cash and cash equivalents	2.00%	2.00%	2.00%
Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 129,598	$ 130,895	$ 122,496
Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	129,598	130,895	122,496
Money market funds \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	13,131	10,190	9,388
Money market funds \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	13,131	10,190	9,388
Insurance contracts* \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	105,371	109,603	101,632
Insurance contracts* \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	105,371	109,603	101,632
Mortgage-backed securities \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	8,471	8,706	8,854
Mortgage-backed securities \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	8,471	8,706	8,854
U.S. Treasury securities \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	2,625	2,396	2,622
U.S. Treasury securities \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 2,625	$ 2,396	$ 2,622

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

+ References

No definition available.

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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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+ Details

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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=123874694&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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