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