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Unpaid losses and loss adjustment expenses - Average historical claims duration (Detail) - Insurance Group [Member]
Dec. 31, 2020
GEICO [Member] | Physical Damage [Member]  
Shortduration Insurance Contracts Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred losses, net of reinsurance, year one 97.00%
Average annual percentage payout of incurred losses, net of reinsurance, year two 2.00%
GEICO [Member] | Auto Liability [Member]  
Shortduration Insurance Contracts Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred losses, net of reinsurance, year one 42.00%
Average annual percentage payout of incurred losses, net of reinsurance, year two 29.00%
Average annual percentage payout of incurred losses, net of reinsurance, year three 13.00%
Average annual percentage payout of incurred losses, net of reinsurance, year four 7.00%
Average annual percentage payout of incurred losses, 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 incurred losses, net of reinsurance, year one 2.00%
Average annual percentage payout of incurred losses, net of reinsurance, year two 7.00%
Average annual percentage payout of incurred losses, net of reinsurance, year three 12.00%
Average annual percentage payout of incurred losses, net of reinsurance, year four 14.00%
Average annual percentage payout of incurred losses, net of reinsurance, year five 14.00%
Average annual percentage payout of incurred losses, net of reinsurance, year six 12.00%
Average annual percentage payout of incurred losses, net of reinsurance, year seven 9.00%
Average annual percentage payout of incurred losses, net of reinsurance, year eight 6.00%
Average annual percentage payout of incurred losses, net of reinsurance, year nine 5.00%
Average annual percentage payout of incurred losses, net of reinsurance, year ten 2.00%
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member]  
Shortduration Insurance Contracts Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred losses, net of reinsurance, year one 16.00%
Average annual percentage payout of incurred losses, net of reinsurance, year two 21.00%
Average annual percentage payout of incurred losses, net of reinsurance, year three 16.00%
Average annual percentage payout of incurred losses, net of reinsurance, year four 13.00%
Average annual percentage payout of incurred losses, net of reinsurance, year five 8.00%
Average annual percentage payout of incurred losses, net of reinsurance, year six 4.00%
Average annual percentage payout of incurred losses, net of reinsurance, year seven 3.00%
Average annual percentage payout of incurred losses, net of reinsurance, year eight 2.00%
Average annual percentage payout of incurred losses, net of reinsurance, year nine 1.00%
Average annual percentage payout of incurred losses, 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 incurred losses, net of reinsurance, year one 19.00%
Average annual percentage payout of incurred losses, net of reinsurance, year two 37.00%
Average annual percentage payout of incurred losses, net of reinsurance, year three 17.00%
Average annual percentage payout of incurred losses, net of reinsurance, year four 8.00%
Average annual percentage payout of incurred losses, net of reinsurance, year five 4.00%
Average annual percentage payout of incurred losses, net of reinsurance, year six 4.00%
Average annual percentage payout of incurred losses, net of reinsurance, year seven 1.00%
Average annual percentage payout of incurred losses, net of reinsurance, year eight 2.00%
Average annual percentage payout of incurred losses, net of reinsurance, year nine 1.00%
Average annual percentage payout of incurred losses, net of reinsurance, year ten 0.00%
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member]  
Shortduration Insurance Contracts Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred losses, net of reinsurance, year one 11.00%
Average annual percentage payout of incurred losses, net of reinsurance, year two 16.00%
Average annual percentage payout of incurred losses, net of reinsurance, year three 14.00%
Average annual percentage payout of incurred losses, net of reinsurance, year four 9.00%
Average annual percentage payout of incurred losses, net of reinsurance, year five 5.00%
Average annual percentage payout of incurred losses, net of reinsurance, year six 5.00%
Average annual percentage payout of incurred losses, net of reinsurance, year seven 5.00%
Average annual percentage payout of incurred losses, net of reinsurance, year eight 2.00%
Average annual percentage payout of incurred losses, net of reinsurance, year nine 2.00%
Average annual percentage payout of incurred losses, net of reinsurance, year ten 1.00%