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, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Reinsurance recoverable $ 3,060 $ 3,201 $ 3,338 $ 3,307
Unpaid losses and loss adjustment expenses 68,458 $ 61,122 $ 53,379 $ 50,519
Insurance Group [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Unpaid losses and ALAE, net 62,661      
Reinsurance recoverable 3,051      
Unpaid unallocated loss adjustment expenses 2,015      
Other unpaid losses and loss adjustment expenses 731      
Unpaid losses and loss adjustment expenses 68,458      
Insurance Group [Member] | GEICO [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Unpaid losses and ALAE, net 17,318      
Reinsurance recoverable 896      
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 9,395      
Reinsurance recoverable 312      
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 19,966      
Reinsurance recoverable 850      
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 7,288      
Reinsurance recoverable 53      
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 8,694      
Reinsurance recoverable $ 940