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, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Reinsurance recoverable	$ 2,855	$ 3,060	$ 3,201	$ 3,338
Unpaid losses and loss adjustment expenses	73,019	$ 68,458	$ 61,122	$ 53,379
Insurance Group [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and ALAE, net	66,529
Reinsurance recoverable	2,785
Unpaid unallocated loss adjustment expenses	2,367
Other unpaid losses and loss adjustment expenses	1,338
Unpaid losses and loss adjustment expenses	73,019
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	321
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	18,475
Reinsurance recoverable	1,014
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	7,479
Reinsurance recoverable	54
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	9,568
Reinsurance recoverable	597
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	9,382
Reinsurance recoverable	268
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	21,304
Reinsurance recoverable	$ 852