Fair value measurements - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Concentration risks, percentage [Abstract]
Percentage in fixed-income and other investments	57.00%	60.00%	61.00%
Percentage investment in common stock of large-cap companies	17.00%	15.00%	15.00%
Percentage investment in target date investments	10.00%	8.00%	7.00%
Percentage investment in common stock of mid-cap companies	8.00%	7.00%	8.00%
Percentage investment in common stock of small-cap companies	4.00%	5.00%	6.00%
Percentage investment in cash and cash equivalents	3.00%	3.00%	2.00%
Percentage investment in international investments	1.00%	1.00%	1.00%
Percentage investment in high-yield investments		1.00%
Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 81,056	$ 84,014	$ 99,886
Fair Value, Inputs, Level 1 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	0	0	0
Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	81,056	84,014	99,886
Fair Value, Inputs, Level 3 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	0	0	0
Money market funds \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	6,985	6,409	5,447
Money market funds \| Fair Value, Inputs, Level 1 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	0	0	0
Money market funds \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	6,985	6,409	5,447
Money market funds \| Fair Value, Inputs, Level 3 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	0	0	0
Insurance contracts \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	62,713	66,283	83,460
Insurance contracts \| Fair Value, Inputs, Level 1 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	0	0	0
Insurance contracts \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	62,713	66,283	83,460
Insurance contracts \| Fair Value, Inputs, Level 3 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	0	0	0
Mortgage-backed securities \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	7,569	7,781	7,999
Mortgage-backed securities \| Fair Value, Inputs, Level 1 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	0	0	0
Mortgage-backed securities \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	7,569	7,781	7,999
Mortgage-backed securities \| Fair Value, Inputs, Level 3 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	0	0	0
U.S. Treasury securities \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	3,789	3,541	2,980
U.S. Treasury securities \| Fair Value, Inputs, Level 1 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	0	0	0
U.S. Treasury securities \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	3,789	3,541	2,980
U.S. Treasury securities \| Fair Value, Inputs, Level 3 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 0	$ 0	$ 0

Concentration Risks, Percentage

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- Definition

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

+ References

No definition available.

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

+ References

No definition available.

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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

No definition available.

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The percentage of investment in high-yield investments.

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Percentage of insurance contracts in international investments.

+ 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
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-3

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