Fair value measurements Assets (Details 4) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022
Concentration risks, percentage [Abstract]
Percentage in fixed-income and other investments	50.00%	63.00%	65.00%
Percentage investment in common stock of large-cap companies	19.00%	15.00%	14.00%
Percentage investment in target date investments	10.00%	6.00%	6.00%
Percentage investment in common stock of mid-cap companies	10.00%	8.00%	7.00%
Percentage investment in common stock of small-cap companies	6.00%	6.00%	6.00%
Percentage investment in cash and cash equivalents	3.00%	2.00%	2.00%
Percentage investment in international investments	1.00%
Percentage Investment in High-Yield Investments	1.00%
Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 375,699	$ 93,701	$ 90,690
Fair Value, Inputs, Level 1 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	276,213
Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	99,486	93,701	90,690
Investment in Knife River \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	276,213
Investment in Knife River \| Fair Value, Inputs, Level 1 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	276,213
Money market funds \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	25,818	4,913	4,878
Money market funds \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	25,818	4,913	4,878
Insurance contracts* \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	62,736	77,958	75,723
Insurance contracts* \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	62,736	77,958	75,723
Mortgage-backed securities \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	7,637	8,294	7,703
Mortgage-backed securities \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	7,637	8,294	7,703
U.S. Treasury securities \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	3,295	2,536	2,386
U.S. Treasury securities \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 3,295	$ 2,536	$ 2,386

Concentration Risks, Percentage

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

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

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

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

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The percentage of investment in high-yield investments.

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Percentage of insurance contracts in international investments.

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Percentage of insurance contracts in cash and cash equivalents.

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The percentage of insurance contracts in target date investments.

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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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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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
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-Publisher FASB
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
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-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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