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, 2019 USD ($) Claim	Dec. 31, 2018 USD ($)	[1]	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]
Claims Development [Line Items]
Unpaid losses and ALAE, net	$ 66,529
GEICO [Member] | Physical Damage [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	17,294
Cumulative Paid Losses and ALAE	16,979
Unpaid losses and ALAE, net	321
GEICO [Member] | Physical Damage [Member] | 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	8,274	$ 8,345
Cumulative Paid Losses and ALAE	8,301	8,078
IBNR and Case Development Liabilities	$ 34
Cumulative Number of Reported Claims | Claim	8,612
GEICO [Member] | Physical Damage [Member] | 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 9,020
Cumulative Paid Losses and ALAE	8,678
IBNR and Case Development Liabilities	$ 334
Cumulative Number of Reported Claims | Claim	8,772
GEICO [Member] | Physical Damage [Member] | 2018-2019 [Member]
Claims Development [Line Items]
Unpaid losses and ALAE, net	$ 315
GEICO [Member] | Physical Damage [Member] | Shortduration Insurance Contracts Accident Years Before2018
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	6
GEICO [Member] | Auto Liability [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	69,046
Cumulative Paid Losses and ALAE	51,079
Unpaid losses and ALAE, net	18,475
GEICO [Member] | Auto Liability [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	10,853	10,824		$ 10,785		$ 10,666		$ 10,590
Cumulative Paid Losses and ALAE	10,472	10,007		9,133		7,694		4,579
IBNR and Case Development Liabilities	$ 156
Cumulative Number of Reported Claims | Claim	2,338
GEICO [Member] | Auto Liability [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 12,178	12,149		12,184		11,800
Cumulative Paid Losses and ALAE	11,294	10,330		8,716		5,069
IBNR and Case Development Liabilities	$ 356
Cumulative Number of Reported Claims | Claim	2,445
GEICO [Member] | Auto Liability [Member] | 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 13,888	13,864		14,095
Cumulative Paid Losses and ALAE	11,799	9,944		5,806
IBNR and Case Development Liabilities	$ 983
Cumulative Number of Reported Claims | Claim	2,628
GEICO [Member] | Auto Liability [Member] | 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 15,226	15,383
Cumulative Paid Losses and ALAE	10,772	6,218
IBNR and Case Development Liabilities	$ 2,425
Cumulative Number of Reported Claims | Claim	2,674
GEICO [Member] | Auto Liability [Member] | 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 16,901
Cumulative Paid Losses and ALAE	6,742
IBNR and Case Development Liabilities	$ 4,694
Cumulative Number of Reported Claims | Claim	2,577
GEICO [Member] | Auto Liability [Member] | 2015-2019 [Member]
Claims Development [Line Items]
Unpaid losses and ALAE, net	$ 17,967
GEICO [Member] | Auto Liability [Member] | Before 2015 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	508
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	12,530
Cumulative Paid Losses and ALAE	5,422
Unpaid losses and ALAE, net	7,479
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2010 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	991	1,006		1,031		1,085		1,140		$ 1,234		$ 1,329		$ 1,348		$ 1,346		$ 1,399
Cumulative Paid Losses and ALAE	888	853		810		745		654		526		377		224		95		15
IBNR and Case Development Liabilities	$ 29
Cumulative Number of Reported Claims | Claim	12
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 968	1,004		1,050		1,115		1,173		1,262		1,321		1,334		1,346
Cumulative Paid Losses and ALAE	822	767		711		632		517		356		200		82		16
IBNR and Case Development Liabilities	$ 38
Cumulative Number of Reported Claims | Claim	11
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 998	1,035		1,078		1,168		1,223		1,277		1,306		1,336
Cumulative Paid Losses and ALAE	789	725		642		522		377		218		93		15
IBNR and Case Development Liabilities	$ 64
Cumulative Number of Reported Claims | Claim	11
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 1,019	1,086		1,127		1,195		1,261		1,296		1,328
Cumulative Paid Losses and ALAE	743	635		518		368		219		90		15
IBNR and Case Development Liabilities	$ 93
Cumulative Number of Reported Claims | Claim	11
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 1,127	1,218		1,246		1,305		1,375		1,370
Cumulative Paid Losses and ALAE	671	540		396		238		106		21
IBNR and Case Development Liabilities	$ 184
Cumulative Number of Reported Claims | Claim	11
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 1,218	1,290		1,269		1,342		1,374
Cumulative Paid Losses and ALAE	543	382		218		108		23
IBNR and Case Development Liabilities	$ 301
Cumulative Number of Reported Claims | Claim	12
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 1,394	1,414		1,416		1,392
Cumulative Paid Losses and ALAE	461	274		115		22
IBNR and Case Development Liabilities	$ 412
Cumulative Number of Reported Claims | Claim	14
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 1,495	1,499		1,466
Cumulative Paid Losses and ALAE	300	128		27
IBNR and Case Development Liabilities	$ 685
Cumulative Number of Reported Claims | Claim	18
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 1,650	1,602
Cumulative Paid Losses and ALAE	166	35
IBNR and Case Development Liabilities	$ 1,088
Cumulative Number of Reported Claims | Claim	18
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 1,670
Cumulative Paid Losses and ALAE	39
IBNR and Case Development Liabilities	$ 1,369
Cumulative Number of Reported Claims | Claim	12
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | Shortduration Insurance Contracts Accident Years2010 Through2019
Claims Development [Line Items]
Unpaid losses and ALAE, net	$ 7,108
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | Shortduration Insurance Contracts Accident Years Before2010
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	371
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	19,080
Cumulative Paid Losses and ALAE	9,915
Unpaid losses and ALAE, net	9,568
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2010 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	528	531		539		548		556		560		577		612		638		662
Cumulative Paid Losses and ALAE	480	472		466		459		445		417		374		314		236		102
IBNR and Case Development Liabilities	$ 36
Cumulative Number of Reported Claims | Claim	41
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 591	596		607		618		621		624		675		675		738
Cumulative Paid Losses and ALAE	512	505		496		481		453		403		333		220		109
IBNR and Case Development Liabilities	$ 56
Cumulative Number of Reported Claims | Claim	46
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 736	750		762		780		791		837		850		873
Cumulative Paid Losses and ALAE	626	611		592		560		501		414		299		116
IBNR and Case Development Liabilities	$ 76
Cumulative Number of Reported Claims | Claim	53
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 1,050	1,072		1,096		1,127		1,178		1,228		1,258
Cumulative Paid Losses and ALAE	858	835		793		725		609		422		177
IBNR and Case Development Liabilities	$ 149
Cumulative Number of Reported Claims | Claim	67
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 1,497	1,482		1,548		1,614		1,638		1,743
Cumulative Paid Losses and ALAE	1,176	1,111		1,007		800		557		239
IBNR and Case Development Liabilities	$ 220
Cumulative Number of Reported Claims | Claim	90
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 2,025	2,014		2,042		2,127		2,169
Cumulative Paid Losses and ALAE	1,488	1,289		1,017		700		289
IBNR and Case Development Liabilities	$ 336
Cumulative Number of Reported Claims | Claim	110
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 2,325	2,359		2,422		2,511
Cumulative Paid Losses and ALAE	1,461	1,148		775		329
IBNR and Case Development Liabilities	$ 533
Cumulative Number of Reported Claims | Claim	114
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 2,842	2,907		3,044
Cumulative Paid Losses and ALAE	1,434	1,003		441
IBNR and Case Development Liabilities	$ 855
Cumulative Number of Reported Claims | Claim	135
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 3,412	3,544
Cumulative Paid Losses and ALAE	1,198	538
IBNR and Case Development Liabilities	$ 1,445
Cumulative Number of Reported Claims | Claim	151
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 4,074
Cumulative Paid Losses and ALAE	682
IBNR and Case Development Liabilities	$ 2,577
Cumulative Number of Reported Claims | Claim	147
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | Shortduration Insurance Contracts Accident Years2010 Through2019
Claims Development [Line Items]
Unpaid losses and ALAE, net	$ 9,165
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | Shortduration Insurance Contracts Accident Years Before2010
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	403
Berkshire Hathaway Reinsurance Group [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	32,426
Cumulative Paid Losses and ALAE	23,414
Unpaid losses and ALAE, net	9,382
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2010 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,062	2,074		2,064		2,085		2,103		2,138		2,226		2,354		2,475		2,516
Cumulative Paid Losses and ALAE	2,043	2,031		2,024		2,000		1,954		1,905		1,742		1,485		1,059		335
IBNR and Case Development Liabilities	23
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	3,674	3,686		3,700		3,723		3,753		3,754		3,851		4,138		4,197
Cumulative Paid Losses and ALAE	3,530	3,512		3,467		3,426		3,336		3,219		2,957		2,305		664
IBNR and Case Development Liabilities	41
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,295	2,311		2,328		2,331		2,384		2,624		2,828		3,132
Cumulative Paid Losses and ALAE	2,163	2,118		2,099		2,023		1,935		1,797		1,219		260
IBNR and Case Development Liabilities	47
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,459	2,510		2,569		2,589		2,679		3,022		3,181
Cumulative Paid Losses and ALAE	2,290	2,251		2,170		2,050		1,854		1,416		513
IBNR and Case Development Liabilities	61
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,035	2,107		2,162		2,306		2,417		2,615
Cumulative Paid Losses and ALAE	1,814	1,764		1,699		1,561		1,235		464
IBNR and Case Development Liabilities	77
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,932	2,935		2,528		3,084		3,243
Cumulative Paid Losses and ALAE	2,239	2,134		1,940		1,591		574
IBNR and Case Development Liabilities	208
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	3,594	3,617		3,892		3,266
Cumulative Paid Losses and ALAE	2,641	2,181		1,790		705
IBNR and Case Development Liabilities	281
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	4,807	4,959		5,258
Cumulative Paid Losses and ALAE	3,638	2,716		1,027
IBNR and Case Development Liabilities	478
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	4,468	4,366
Cumulative Paid Losses and ALAE	2,309	907
IBNR and Case Development Liabilities	1,025
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	4,100
Cumulative Paid Losses and ALAE	747
IBNR and Case Development Liabilities	1,977
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | Shortduration Insurance Contracts Accident Years2010 Through2019
Claims Development [Line Items]
Unpaid losses and ALAE, net	9,012
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | Shortduration Insurance Contracts Accident Years Before2010
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	370
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	23,921
Cumulative Paid Losses and ALAE	11,047
Unpaid losses and ALAE, net	21,304
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2010 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	1,913	1,958		1,878		2,040		2,085		2,135		2,253		2,316		2,383		2,296
Cumulative Paid Losses and ALAE	1,552	1,526		1,478		1,433		1,369		1,274		1,022		834		542		$ 117
IBNR and Case Development Liabilities	203
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,240	2,274		2,313		2,320		2,411		2,500		2,560		2,690		2,602
Cumulative Paid Losses and ALAE	1,727	1,693		1,653		1,575		1,483		1,395		1,155		812		$ 289
IBNR and Case Development Liabilities	299
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,554	2,611		2,678		2,790		2,861		2,797		2,962		2,784
Cumulative Paid Losses and ALAE	1,805	1,746		1,646		1,522		1,365		1,136		745		$ 307
IBNR and Case Development Liabilities	318
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	1,928	2,023		2,077		2,131		2,287		2,257		2,124
Cumulative Paid Losses and ALAE	1,195	1,135		1,034		930		802		517		$ 290
IBNR and Case Development Liabilities	411
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	1,942	1,904		1,990		2,027		2,058		1,863
Cumulative Paid Losses and ALAE	955	871		748		639		474		$ 149
IBNR and Case Development Liabilities	574
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	1,872	1,999		2,098		2,071		1,870
Cumulative Paid Losses and ALAE	921	830		710		487		$ 195
IBNR and Case Development Liabilities	534
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	1,971	2,014		2,106		1,900
Cumulative Paid Losses and ALAE	860	730		553		$ 252
IBNR and Case Development Liabilities	680
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,552	2,674		2,186
Cumulative Paid Losses and ALAE	816	562		$ 230
IBNR and Case Development Liabilities	941
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	3,544	2,914
Cumulative Paid Losses and ALAE	865	$ 264
IBNR and Case Development Liabilities	1,511
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	3,405
Cumulative Paid Losses and ALAE	351
IBNR and Case Development Liabilities	2,348
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | Shortduration Insurance Contracts Accident Years2010 Through2019
Claims Development [Line Items]
Unpaid losses and ALAE, net	12,874
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | Shortduration Insurance Contracts Accident Years Before2010
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	$ 8,430

[1] Unaudited required supplemental information