Unpaid losses and loss adjustment expenses - Incurred and paid losses and allocated loss adjustment expenses (Detail) - Insurance Group [Member]
Claim in Thousands, $ in Millions
Dec. 31, 2018
USD ($)
Claim
Dec. 31, 2017
USD ($)
 [1]
Dec. 31, 2016
USD ($)
 [1]
Dec. 31, 2015
USD ($)
 [1]
Dec. 31, 2014
USD ($)
 [1]
Dec. 31, 2013
USD ($)
 [1]
Dec. 31, 2012
USD ($)
 [1]
Dec. 31, 2011
USD ($)
 [1]
Dec. 31, 2010
USD ($)
 [1]
Dec. 31, 2009
USD ($)
 [1]
Claims Development [Line Items]                    
Net unpaid losses and ALAE $ 62,661                  
GEICO [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 97,431                  
Cumulative Paid Losses and ALAE 80,699                  
Net unpaid losses and ALAE 17,318                  
GEICO [Member] | 2014 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 14,651 $ 14,589 $ 14,559 $ 14,572 $ 14,680          
IBNR and Case Development Liabilities $ 148                  
Cumulative Number of Reported Claims | Claim 7,976                  
Cumulative Paid Losses and ALAE $ 14,273 13,850 13,140 12,036 9,199          
GEICO [Member] | 2015 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 17,039 16,993 16,875 16,887            
IBNR and Case Development Liabilities $ 319                  
Cumulative Number of Reported Claims | Claim 8,926                  
Cumulative Paid Losses and ALAE $ 16,186 15,285 13,858 10,606            
GEICO [Member] | 2016 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 19,364 19,390 19,106              
IBNR and Case Development Liabilities $ 823                  
Cumulative Number of Reported Claims | Claim 9,633                  
Cumulative Paid Losses and ALAE $ 17,493 15,862 12,020              
GEICO [Member] | 2017 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 22,257 22,675                
IBNR and Case Development Liabilities $ 2,208                  
Cumulative Number of Reported Claims | Claim 10,816                  
Cumulative Paid Losses and ALAE $ 18,249 13,878                
GEICO [Member] | 2018 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 24,120                  
IBNR and Case Development Liabilities $ 4,659                  
Cumulative Number of Reported Claims | Claim 10,968                  
Cumulative Paid Losses and ALAE $ 14,498                  
GEICO [Member] | 2014-2018 [Member]                    
Claims Development [Line Items]                    
Net unpaid losses and ALAE 16,732                  
GEICO [Member] | Before 2014 [Member]                    
Claims Development [Line Items]                    
Net unpaid losses and ALAE for accident years before earliest accident year presented 586                  
Berkshire Hathaway Reinsurance Group [Member] | Property [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 30,598                  
Cumulative Paid Losses and ALAE 21,618                  
Net unpaid losses and ALAE 9,395                  
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2014 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 2,115 2,171 2,315 2,427 2,621          
IBNR and Case Development Liabilities 120                  
Cumulative Paid Losses and ALAE 1,773 1,708 1,569 1,244 465          
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2015 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 2,938 2,531 3,088 3,244            
IBNR and Case Development Liabilities 136                  
Cumulative Paid Losses and ALAE 2,135 1,940 1,590 569            
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2016 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 3,625 3,906 3,267              
IBNR and Case Development Liabilities 583                  
Cumulative Paid Losses and ALAE 2,187 1,792 703              
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2017 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 4,958 5,248                
IBNR and Case Development Liabilities 600                  
Cumulative Paid Losses and ALAE 2,713 1,026                
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2018 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 4,373                  
IBNR and Case Development Liabilities 2,125                  
Cumulative Paid Losses and ALAE 910                  
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2009 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 1,984 1,984 1,985 1,990 2,031 $ 2,058 $ 2,154 $ 2,232 $ 2,037 $ 2,366
IBNR and Case Development Liabilities 7                  
Cumulative Paid Losses and ALAE 1,963 1,961 1,959 1,950 1,916 1,846 1,777 1,613 1,110 402
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2010 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 2,114 2,106 2,126 2,145 2,180 2,274 2,408 2,380 2,523  
IBNR and Case Development Liabilities 23                  
Cumulative Paid Losses and ALAE 2,065 2,058 2,033 1,987 1,931 1,772 1,506 1,024 336  
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2011 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 3,652 3,666 3,689 3,719 3,720 3,810 4,084 4,300    
IBNR and Case Development Liabilities 52                  
Cumulative Paid Losses and ALAE 3,485 3,440 3,398 3,310 3,200 2,931 2,282 701    
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2012 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 2,315 2,333 2,336 2,389 2,629 2,833 3,144      
IBNR and Case Development Liabilities 65                  
Cumulative Paid Losses and ALAE 2,123 2,103 2,028 1,939 1,801 1,222 266      
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2013 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 2,524 2,584 2,604 2,695 3,039 3,196        
IBNR and Case Development Liabilities 105                  
Cumulative Paid Losses and ALAE 2,264 2,184 2,063 1,866 1,427 516        
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2009-2018 [Member]                    
Claims Development [Line Items]                    
Net unpaid losses and ALAE 8,980                  
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | Before 2009 [Member]                    
Claims Development [Line Items]                    
Net unpaid losses and ALAE for accident years before earliest accident year presented 415                  
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 22,609                  
Cumulative Paid Losses and ALAE 11,113                  
Net unpaid losses and ALAE 19,966                  
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2014 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 1,905 1,991 2,029 2,061 1,867          
IBNR and Case Development Liabilities 611                  
Cumulative Paid Losses and ALAE 875 753 644 478 150          
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2015 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 2,000 2,099 2,072 1,875            
IBNR and Case Development Liabilities 647                  
Cumulative Paid Losses and ALAE 834 714 491 196            
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2016 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 2,014 2,107 1,903              
IBNR and Case Development Liabilities 818                  
Cumulative Paid Losses and ALAE 734 556 253              
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2017 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 2,676 2,189                
IBNR and Case Development Liabilities 1,141                  
Cumulative Paid Losses and ALAE 568 231                
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2018 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 2,917                  
IBNR and Case Development Liabilities 1,856                  
Cumulative Paid Losses and ALAE 267                  
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2009 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 2,209 2,192 2,231 2,292 2,339 2,414 2,476 2,546 2,687 2,375
IBNR and Case Development Liabilities 213                  
Cumulative Paid Losses and ALAE 1,716 1,684 1,658 1,622 1,586 1,430 1,211 940 852 247
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2010 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 1,963 1,883 2,044 2,090 2,139 2,258 2,322 2,387 2,301  
IBNR and Case Development Liabilities 228                  
Cumulative Paid Losses and ALAE 1,531 1,482 1,437 1,374 1,278 1,026 838 546 118  
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2011 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 2,282 2,320 2,327 2,418 2,508 2,567 2,696 2,609    
IBNR and Case Development Liabilities 363                  
Cumulative Paid Losses and ALAE 1,698 1,659 1,581 1,489 1,400 1,160 817 292    
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2012 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 2,615 2,682 2,793 2,864 2,799 2,964 2,789      
IBNR and Case Development Liabilities 514                  
Cumulative Paid Losses and ALAE 1,750 1,651 1,527 1,370 1,140 749 309      
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2013 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 2,028 2,082 2,134 2,290 2,259 2,130        
IBNR and Case Development Liabilities 544                  
Cumulative Paid Losses and ALAE 1,140 1,039 935 807 521 291        
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2009-2018 [Member]                    
Claims Development [Line Items]                    
Net unpaid losses and ALAE 11,496                  
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | Before 2009 [Member]                    
Claims Development [Line Items]                    
Net unpaid losses and ALAE for accident years before earliest accident year presented 8,470                  
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 12,058                  
Cumulative Paid Losses and ALAE 5,149                  
Net unpaid losses and ALAE 7,288                  
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2014 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 1,218 1,246 1,305 1,375 1,370          
IBNR and Case Development Liabilities $ 296                  
Cumulative Number of Reported Claims | Claim 13                  
Cumulative Paid Losses and ALAE $ 540 396 238 106 21          
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2015 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 1,290 1,269 1,342 1,374            
IBNR and Case Development Liabilities $ 450                  
Cumulative Number of Reported Claims | Claim 14                  
Cumulative Paid Losses and ALAE $ 382 218 108 23            
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2016 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 1,414 1,416 1,392              
IBNR and Case Development Liabilities $ 641                  
Cumulative Number of Reported Claims | Claim 14                  
Cumulative Paid Losses and ALAE $ 274 115 22              
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2017 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 1,499 1,466                
IBNR and Case Development Liabilities $ 1,011                  
Cumulative Number of Reported Claims | Claim 18                  
Cumulative Paid Losses and ALAE $ 128 27                
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2018 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 1,602                  
IBNR and Case Development Liabilities $ 1,410                  
Cumulative Number of Reported Claims | Claim 15                  
Cumulative Paid Losses and ALAE $ 35                  
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2009 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 904 922 941 988 1,060 1,153 1,251 1,294 1,349 1,423
IBNR and Case Development Liabilities $ 22                  
Cumulative Number of Reported Claims | Claim 13                  
Cumulative Paid Losses and ALAE $ 810 789 753 699 619 480 350 203 90 5
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2010 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 1,006 1,031 1,085 1,140 1,234 1,329 1,348 1,346 1,399  
IBNR and Case Development Liabilities $ 43                  
Cumulative Number of Reported Claims | Claim 13                  
Cumulative Paid Losses and ALAE $ 853 810 745 654 526 377 224 95 15  
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2011 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 1,004 1,050 1,115 1,173 1,262 1,321 1,334 1,346    
IBNR and Case Development Liabilities $ 71                  
Cumulative Number of Reported Claims | Claim 13                  
Cumulative Paid Losses and ALAE $ 767 711 632 517 356 200 82 16    
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2012 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 1,035 1,078 1,168 1,223 1,277 1,306 1,336      
IBNR and Case Development Liabilities $ 108                  
Cumulative Number of Reported Claims | Claim 13                  
Cumulative Paid Losses and ALAE $ 725 642 522 377 218 93 15      
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2013 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 1,086 1,127 1,195 1,261 1,296 1,328        
IBNR and Case Development Liabilities $ 160                  
Cumulative Number of Reported Claims | Claim 12                  
Cumulative Paid Losses and ALAE $ 635 518 368 219 90 15        
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2009-2018 [Member]                    
Claims Development [Line Items]                    
Net unpaid losses and ALAE 6,909                  
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | Before 2009 [Member]                    
Claims Development [Line Items]                    
Net unpaid losses and ALAE for accident years before earliest accident year presented 379                  
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 16,718                  
Cumulative Paid Losses and ALAE 8,346                  
Net unpaid losses and ALAE 8,694                  
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2014 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 1,572 1,641 1,716 1,737 1,866          
IBNR and Case Development Liabilities $ 287                  
Cumulative Number of Reported Claims | Claim 112                  
Cumulative Paid Losses and ALAE $ 1,155 1,046 837 581 247          
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2015 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 2,124 2,156 2,244 2,314            
IBNR and Case Development Liabilities $ 445                  
Cumulative Number of Reported Claims | Claim 133                  
Cumulative Paid Losses and ALAE $ 1,328 1,044 715 297            
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2016 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 2,475 2,561 2,656              
IBNR and Case Development Liabilities $ 833                  
Cumulative Number of Reported Claims | Claim 127                  
Cumulative Paid Losses and ALAE $ 1,181 792 332              
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2017 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 3,122 3,262                
IBNR and Case Development Liabilities $ 1,395                  
Cumulative Number of Reported Claims | Claim 143                  
Cumulative Paid Losses and ALAE $ 1,078 467                
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2018 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 3,761                  
IBNR and Case Development Liabilities $ 2,459                  
Cumulative Number of Reported Claims | Claim 142                  
Cumulative Paid Losses and ALAE $ 570                  
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2009 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 562 576 585 589 595 609 632 676 673 732
IBNR and Case Development Liabilities $ 40                  
Cumulative Number of Reported Claims | Claim 46                  
Cumulative Paid Losses and ALAE $ 507 503 499 488 464 434 389 328 228 $ 99
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2010 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 553 562 576 584 589 609 644 675 715  
IBNR and Case Development Liabilities $ 42                  
Cumulative Number of Reported Claims | Claim 47                  
Cumulative Paid Losses and ALAE $ 491 484 479 465 437 393 327 250 $ 106  
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2011 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 622 632 647 653 657 709 706 786    
IBNR and Case Development Liabilities $ 72                  
Cumulative Number of Reported Claims | Claim 51                  
Cumulative Paid Losses and ALAE $ 522 512 498 470 415 344 222 $ 114    
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2012 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 779 792 815 828 876 887 907      
IBNR and Case Development Liabilities $ 105                  
Cumulative Number of Reported Claims | Claim 63                  
Cumulative Paid Losses and ALAE $ 627 607 576 515 422 301 $ 101      
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2013 [Member]                    
Claims Development [Line Items]                    
Incurred Losses and ALAE 1,148 1,173 1,210 1,257 1,310 1,339        
IBNR and Case Development Liabilities $ 188                  
Cumulative Number of Reported Claims | Claim 83                  
Cumulative Paid Losses and ALAE $ 887 $ 842 $ 776 $ 650 $ 452 $ 193        
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2009-2018 [Member]                    
Claims Development [Line Items]                    
Net unpaid losses and ALAE 8,372                  
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | Before 2009 [Member]                    
Claims Development [Line Items]                    
Net unpaid losses and ALAE for accident years before earliest accident year presented $ 322                  
[1] Unaudited supplemental information