Unpaid losses and loss adjustment expenses - Reconciliation of unpaid losses and allocated loss adjustment expenses to balance sheet liability (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Reinsurance recoverable	$ 2,912	$ 2,855	$ 3,060	$ 3,201
Unpaid losses and loss adjustment expenses	79,854	$ 73,019	$ 68,458	$ 61,122
Insurance Group [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and ALAE, net	72,640
Reinsurance recoverable	2,824
Unpaid unallocated loss adjustment expenses	2,671
Other unpaid losses and loss adjustment expenses	1,719
Unpaid losses and loss adjustment expenses	79,854
Insurance Group [Member] | GEICO [Member] | Physical Damage [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and ALAE, net	524
Insurance Group [Member] | GEICO [Member] | Auto Liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and ALAE, net	18,755
Reinsurance recoverable	1,109
Insurance Group [Member] | 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	7,897
Reinsurance recoverable	49
Insurance Group [Member] | 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	11,294
Reinsurance recoverable	621
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Property [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and ALAE, net	11,280
Reinsurance recoverable	181
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and ALAE, net	22,890
Reinsurance recoverable	$ 864