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Unpaid losses and loss adjustment expenses - Average historical claims duration (Detail) - Insurance Group [Member]
Dec. 31, 2017
GEICO [Member]  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred losses, net of reinsurance, year one 61.90%
Average annual percentage payout of incurred losses, net of reinsurance, year two 19.60%
Average annual percentage payout of incurred losses, net of reinsurance, year three 8.10%
Average annual percentage payout of incurred losses, net of reinsurance, year four 4.90%
Average annual percentage payout of incurred losses, net of reinsurance, year five 2.80%
Berkshire Hathaway Reinsurance Group [Member] | Property [Member]  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred losses, net of reinsurance, year one 18.70%
Average annual percentage payout of incurred losses, net of reinsurance, year two 37.50%
Average annual percentage payout of incurred losses, net of reinsurance, year three 18.80%
Average annual percentage payout of incurred losses, net of reinsurance, year four 8.30%
Average annual percentage payout of incurred losses, net of reinsurance, year five 4.50%
Average annual percentage payout of incurred losses, net of reinsurance, year six 2.70%
Average annual percentage payout of incurred losses, net of reinsurance, year seven 1.60%
Average annual percentage payout of incurred losses, net of reinsurance, year eight 1.40%
Average annual percentage payout of incurred losses, net of reinsurance, year nine 0.30%
Average annual percentage payout of incurred losses, net of reinsurance, year ten 0.00%
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member]  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred losses, net of reinsurance, year one 10.70%
Average annual percentage payout of incurred losses, net of reinsurance, year two 18.40%
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 8.30%
Average annual percentage payout of incurred losses, net of reinsurance, year five 7.10%
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 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 1.50%
Average annual percentage payout of incurred losses, net of reinsurance, year ten 1.80%
Berkshire Hathaway Primary Group [Member]  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred losses, net of reinsurance, year one 14.60%
Average annual percentage payout of incurred losses, net of reinsurance, year two 17.70%
Average annual percentage payout of incurred losses, net of reinsurance, year three 15.00%
Average annual percentage payout of incurred losses, net of reinsurance, year four 12.50%
Average annual percentage payout of incurred losses, net of reinsurance, year five 9.80%
Average annual percentage payout of incurred losses, net of reinsurance, year six 7.30%
Average annual percentage payout of incurred losses, net of reinsurance, year seven 4.40%
Average annual percentage payout of incurred losses, net of reinsurance, year eight 2.70%
Average annual percentage payout of incurred losses, net of reinsurance, year nine 1.40%
Average annual percentage payout of incurred losses, net of reinsurance, year ten 1.70%