Unpaid losses and loss adjustment expenses - Average historical claims duration (Detail) - Insurance Group [Member]	Dec. 31, 2024
GEICO [Member] \| Physical Damage [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
Average annual percentage payout of losses incurred, net of reinsurance, year one	97.00%
Average annual percentage payout of losses incurred, net of reinsurance, year two	3.00%
GEICO [Member] \| Auto Liability [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
Average annual percentage payout of losses incurred, net of reinsurance, year one	41.00%
Average annual percentage payout of losses incurred, net of reinsurance, year two	31.00%
Average annual percentage payout of losses incurred, net of reinsurance, year three	13.00%
Average annual percentage payout of losses incurred, net of reinsurance, year four	7.00%
Average annual percentage payout of losses incurred, net of reinsurance, year five	4.00%
Berkshire Hathaway Primary Group [Member] \| Medical Professional Liability [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
Average annual percentage payout of losses incurred, net of reinsurance, year one	2.00%
Average annual percentage payout of losses incurred, net of reinsurance, year two	8.00%
Average annual percentage payout of losses incurred, net of reinsurance, year three	12.00%
Average annual percentage payout of losses incurred, net of reinsurance, year four	14.00%
Average annual percentage payout of losses incurred, net of reinsurance, year five	13.00%
Average annual percentage payout of losses incurred, net of reinsurance, year six	12.00%
Average annual percentage payout of losses incurred, net of reinsurance, year seven	8.00%
Average annual percentage payout of losses incurred, net of reinsurance, year eight	8.00%
Average annual percentage payout of losses incurred, net of reinsurance, year nine	4.00%
Average annual percentage payout of losses incurred, net of reinsurance, year ten	4.00%
Berkshire Hathaway Primary Group [Member] \| Workers' Compensation and Other Casualty [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
Average annual percentage payout of losses incurred, net of reinsurance, year one	14.00%
Average annual percentage payout of losses incurred, net of reinsurance, year two	17.00%
Average annual percentage payout of losses incurred, net of reinsurance, year three	14.00%
Average annual percentage payout of losses incurred, net of reinsurance, year four	12.00%
Average annual percentage payout of losses incurred, net of reinsurance, year five	10.00%
Average annual percentage payout of losses incurred, net of reinsurance, year six	7.00%
Average annual percentage payout of losses incurred, net of reinsurance, year seven	5.00%
Average annual percentage payout of losses incurred, net of reinsurance, year eight	3.00%
Average annual percentage payout of losses incurred, net of reinsurance, year nine	1.00%
Average annual percentage payout of losses incurred, net of reinsurance, year ten	1.00%
Berkshire Hathaway Reinsurance Group [Member] \| Property [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
Average annual percentage payout of losses incurred, net of reinsurance, year one	22.00%
Average annual percentage payout of losses incurred, net of reinsurance, year two	35.00%
Average annual percentage payout of losses incurred, net of reinsurance, year three	15.00%
Average annual percentage payout of losses incurred, net of reinsurance, year four	8.00%
Average annual percentage payout of losses incurred, net of reinsurance, year five	4.00%
Average annual percentage payout of losses incurred, net of reinsurance, year six	4.00%
Average annual percentage payout of losses incurred, net of reinsurance, year seven	2.00%
Average annual percentage payout of losses incurred, net of reinsurance, year eight	1.00%
Average annual percentage payout of losses incurred, net of reinsurance, year nine	2.00%
Average annual percentage payout of losses incurred, net of reinsurance, year ten	2.00%
Berkshire Hathaway Reinsurance Group [Member] \| Casualty [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
Average annual percentage payout of losses incurred, net of reinsurance, year one	13.00%
Average annual percentage payout of losses incurred, net of reinsurance, year two	17.00%
Average annual percentage payout of losses incurred, net of reinsurance, year three	13.00%
Average annual percentage payout of losses incurred, net of reinsurance, year four	11.00%
Average annual percentage payout of losses incurred, net of reinsurance, year five	8.00%
Average annual percentage payout of losses incurred, net of reinsurance, year six	6.00%
Average annual percentage payout of losses incurred, net of reinsurance, year seven	5.00%
Average annual percentage payout of losses incurred, net of reinsurance, year eight	4.00%
Average annual percentage payout of losses incurred, net of reinsurance, year nine	4.00%
Average annual percentage payout of losses incurred, net of reinsurance, year ten	3.00%

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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- Definition

Percentage of average annual payout, after reinsurance, in the eighth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 9F
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480046/944-40-55-9F

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 4G
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-4G

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- Definition

Percentage of average annual payout, after reinsurance, in the fifth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

+ References

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- Definition

Percentage of average annual payout, after reinsurance, in the fourth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

+ References

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- Definition

Percentage of average annual payout, after reinsurance, in the ninth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

+ References

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- Definition

Percentage of average annual payout, after reinsurance, in the first year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

+ References

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- Definition

Percentage of average annual payout, after reinsurance, in the seventh year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

+ References

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Percentage of average annual payout, after reinsurance, in the sixth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

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- Definition

Percentage of average annual payout, after reinsurance, in the tenth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

+ References

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- Definition

Percentage of average annual payout, after reinsurance, in the third year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

+ References

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Percentage of average annual payout, after reinsurance, in the second year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

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