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, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and ALAE, net	$ 55,579
Reinsurance recoverable	3,201	$ 3,338	$ 3,307	$ 3,116
Unpaid unallocated loss adjustment expenses	2,342
Unpaid losses and loss adjustment expenses	61,122	$ 53,379	$ 50,519	$ 48,208
Insurance Group [Member] | GEICO [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and ALAE, net	15,655
Reinsurance recoverable	859
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	8,838
Reinsurance recoverable	289
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	19,219
Reinsurance recoverable	1,028
Insurance Group [Member] | Berkshire Hathaway Primary Group [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and ALAE, net	11,867
Reinsurance recoverable	$ 1,025