Unpaid losses and loss adjustment expenses - Reconciliation of unpaid losses and allocated loss adjustment expenses to balance sheet liability (Detail) - Insurance Group [Member] $ in Millions	Dec. 31, 2024 USD ($)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and ALAE, net	$ 104,671
Reinsurance recoverable	4,392
Unallocated loss adjustment expenses	2,414
Other losses and loss adjustment expenses	3,674
Unpaid losses and loss adjustment expenses	115,151
GEICO [Member] | Physical Damage [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and ALAE, net	885
Reinsurance recoverable	3
GEICO [Member] | Auto Liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and ALAE, net	20,818
Reinsurance recoverable	884
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and ALAE, net	9,432
Reinsurance recoverable	29
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and ALAE, net	24,335
Reinsurance recoverable	1,473
Berkshire Hathaway Reinsurance Group [Member] | Property [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and ALAE, net	15,851
Reinsurance recoverable	634
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and ALAE, net	33,350
Reinsurance recoverable	$ 1,369