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, 2020 USD ($) Claim	Dec. 31, 2019 USD ($)		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]
Claims Development [Line Items]
Unpaid losses and ALAE, net	$ 72,640
GEICO [Member] | Physical Damage [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	17,523
Cumulative Paid Losses and ALAE	17,023
Unpaid losses and ALAE, net	524
GEICO [Member] | Physical Damage [Member] | 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	8,920	$ 9,020
Cumulative Paid Losses and ALAE	8,905	8,678
IBNR and Case Development Liabilities	$ 69
Cumulative Number of Reported Claims | Claim	8,929
GEICO [Member] | Physical Damage [Member] | 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 8,603
Cumulative Paid Losses and ALAE	8,118
IBNR and Case Development Liabilities	$ 296
Cumulative Number of Reported Claims | Claim	7,794
GEICO [Member] | Physical Damage [Member] | 2019-2020 [Member]
Claims Development [Line Items]
Unpaid losses and ALAE, net	$ 500
GEICO [Member] | Physical Damage [Member] | Shortduration Insurance Contracts Accident Years Before 2019
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	24
GEICO [Member] | Auto Liability [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	72,322
Cumulative Paid Losses and ALAE	54,171
Unpaid losses and ALAE, net	18,755
GEICO [Member] | Auto Liability [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	12,198	12,178	[1]	$ 12,149		$ 12,184		$ 11,800
Cumulative Paid Losses and ALAE	11,718	11,294	[1]	10,330		8,716		5,069
IBNR and Case Development Liabilities	$ 222
Cumulative Number of Reported Claims | Claim	2,451
GEICO [Member] | Auto Liability [Member] | 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 13,824	13,888	[1]	13,864		14,095
Cumulative Paid Losses and ALAE	12,729	11,799	[1]	9,944		5,806
IBNR and Case Development Liabilities	$ 502
Cumulative Number of Reported Claims | Claim	2,639
GEICO [Member] | Auto Liability [Member] | 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 16,678	16,901	[1]
Cumulative Paid Losses and ALAE	11,671	6,742	[1]
IBNR and Case Development Liabilities	$ 2,905
Cumulative Number of Reported Claims | Claim	2,749
GEICO [Member] | Auto Liability [Member] | 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 14,637
Cumulative Paid Losses and ALAE	5,395
IBNR and Case Development Liabilities	$ 4,482
Cumulative Number of Reported Claims | Claim	1,945
GEICO [Member] | Auto Liability [Member] | 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 14,985	15,226	[1]	15,383
Cumulative Paid Losses and ALAE	12,658	10,772	[1]	6,218
IBNR and Case Development Liabilities	$ 1,163
Cumulative Number of Reported Claims | Claim	2,702
GEICO [Member] | Auto Liability [Member] | 2016-2020 [Member]
Claims Development [Line Items]
Unpaid losses and ALAE, net	$ 18,151
GEICO [Member] | Auto Liability [Member] | Before 2016 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	604
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	13,032
Cumulative Paid Losses and ALAE	5,518
Unpaid losses and ALAE, net	7,897
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	972	968	[1]	1,004		1,050		1,115		$ 1,173		$ 1,262		$ 1,321		$ 1,334		$ 1,346
Cumulative Paid Losses and ALAE	842	822	[1]	767		711		632		517		356		200		82		16
IBNR and Case Development Liabilities	$ 39
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	$ 988	998	[1]	1,035		1,078		1,168		1,223		1,277		1,306		1,336
Cumulative Paid Losses and ALAE	830	789	[1]	725		642		522		377		218		93		15
IBNR and Case Development Liabilities	$ 53
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	$ 985	1,019	[1]	1,086		1,127		1,195		1,261		1,296		1,328
Cumulative Paid Losses and ALAE	793	743	[1]	635		518		368		219		90		15
IBNR and Case Development Liabilities	$ 65
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	$ 1,061	1,127	[1]	1,218		1,246		1,305		1,375		1,370
Cumulative Paid Losses and ALAE	752	671	[1]	540		396		238		106		21
IBNR and Case Development Liabilities	$ 129
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	$ 1,157	1,218	[1]	1,290		1,269		1,342		1,374
Cumulative Paid Losses and ALAE	663	543	[1]	382		218		108		23
IBNR and Case Development Liabilities	$ 202
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	$ 1,341	1,394	[1]	1,414		1,416		1,392
Cumulative Paid Losses and ALAE	620	461	[1]	274		115		22
IBNR and Case Development Liabilities	$ 286
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	$ 1,474	1,495	[1]	1,499		1,466
Cumulative Paid Losses and ALAE	457	300	[1]	128		27
IBNR and Case Development Liabilities	$ 494
Cumulative Number of Reported Claims | Claim	20
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 1,691	1,670	[1]
Cumulative Paid Losses and ALAE	160	39	[1]
IBNR and Case Development Liabilities	$ 1,204
Cumulative Number of Reported Claims | Claim	17
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 1,704
Cumulative Paid Losses and ALAE	34
IBNR and Case Development Liabilities	$ 1,529
Cumulative Number of Reported Claims | Claim	13
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 1,659	1,650	[1]	1,602
Cumulative Paid Losses and ALAE	367	166	[1]	35
IBNR and Case Development Liabilities	$ 780
Cumulative Number of Reported Claims | Claim	22
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | Shortduration Insurance Contracts Accident Years 2011 Through 2020
Claims Development [Line Items]
Unpaid losses and ALAE, net	$ 7,514
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | Shortduration Insurance Contracts Accident Years Before 2011
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	383
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	23,007
Cumulative Paid Losses and ALAE	12,099
Unpaid losses and ALAE, net	11,294
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	576	591	[1]	596		607		618		621		624		675		675		738
Cumulative Paid Losses and ALAE	519	512	[1]	505		496		481		453		403		333		220		109
IBNR and Case Development Liabilities	$ 39
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	$ 718	736	[1]	750		762		780		791		837		850		873
Cumulative Paid Losses and ALAE	634	626	[1]	611		592		560		501		414		299		116
IBNR and Case Development Liabilities	$ 53
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	$ 1,028	1,050	[1]	1,072		1,096		1,127		1,178		1,228		1,258
Cumulative Paid Losses and ALAE	874	858	[1]	835		793		725		609		422		177
IBNR and Case Development Liabilities	$ 120
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	$ 1,477	1,497	[1]	1,482		1,548		1,614		1,638		1,743
Cumulative Paid Losses and ALAE	1,214	1,176	[1]	1,111		1,007		800		557		239
IBNR and Case Development Liabilities	$ 190
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	$ 1,997	2,025	[1]	2,014		2,042		2,127		2,169
Cumulative Paid Losses and ALAE	1,570	1,488	[1]	1,289		1,017		700		289
IBNR and Case Development Liabilities	$ 267
Cumulative Number of Reported Claims | Claim	111
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 2,365	2,325	[1]	2,359		2,422		2,511
Cumulative Paid Losses and ALAE	1,661	1,461	[1]	1,148		775		329
IBNR and Case Development Liabilities	$ 470
Cumulative Number of Reported Claims | Claim	115
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 2,843	2,842	[1]	2,907		3,044
Cumulative Paid Losses and ALAE	1,771	1,434	[1]	1,003		441
IBNR and Case Development Liabilities	$ 691
Cumulative Number of Reported Claims | Claim	138
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 4,102	4,074	[1]
Cumulative Paid Losses and ALAE	1,478	682	[1]
IBNR and Case Development Liabilities	$ 1,788
Cumulative Number of Reported Claims | Claim	170
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 4,421
Cumulative Paid Losses and ALAE	695
IBNR and Case Development Liabilities	$ 2,987
Cumulative Number of Reported Claims | Claim	120
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	$ 3,480	3,412	[1]	3,544
Cumulative Paid Losses and ALAE	1,683	1,198	[1]	538
IBNR and Case Development Liabilities	$ 1,152
Cumulative Number of Reported Claims | Claim	160
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | Shortduration Insurance Contracts Accident Years 2011 Through 2020
Claims Development [Line Items]
Unpaid losses and ALAE, net	$ 10,908
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | Shortduration Insurance Contracts Accident Years Before 2011
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	386
Berkshire Hathaway Reinsurance Group [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	36,343
Cumulative Paid Losses and ALAE	25,398
Unpaid losses and ALAE, net	11,280
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	3,616	3,627	[1]	3,638		3,654		3,672		3,707		3,711		3,804		4,095		4,111
Cumulative Paid Losses and ALAE	3,507	3,493	[1]	3,474		3,429		3,387		3,304		3,188		2,917		2,259		609
IBNR and Case Development Liabilities	30
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,305	2,315	[1]	2,329		2,348		2,351		2,403		2,644		2,846		3,153
Cumulative Paid Losses and ALAE	2,199	2,181	[1]	2,135		2,117		2,040		1,950		1,813		1,232		262
IBNR and Case Development Liabilities	35
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,504	2,518	[1]	2,570		2,631		2,653		2,745		3,093		3,255
Cumulative Paid Losses and ALAE	2,376	2,347	[1]	2,307		2,226		2,105		1,906		1,459		526
IBNR and Case Development Liabilities	45
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,021	2,050	[1]	2,123		2,178		2,322		2,436		2,648
Cumulative Paid Losses and ALAE	1,858	1,829	[1]	1,779		1,713		1,574		1,249		467
IBNR and Case Development Liabilities	48
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	3,000	2,976	[1]	2,979		2,577		3,135		3,287
Cumulative Paid Losses and ALAE	2,453	2,271	[1]	2,166		1,969		1,614		581
IBNR and Case Development Liabilities	143
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	3,616	3,614	[1]	3,646		3,923		3,293
Cumulative Paid Losses and ALAE	2,923	2,670	[1]	2,208		1,811		709
IBNR and Case Development Liabilities	218
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	4,727	4,837	[1]	4,986		5,291
Cumulative Paid Losses and ALAE	3,972	3,660	[1]	2,734		1,028
IBNR and Case Development Liabilities	187
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	4,299	4,146	[1]
Cumulative Paid Losses and ALAE	2,282	751	[1]
IBNR and Case Development Liabilities	952
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	5,858
Cumulative Paid Losses and ALAE	960
IBNR and Case Development Liabilities	3,129
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	4,397	4,524	[1]	4,426
Cumulative Paid Losses and ALAE	2,868	2,341	[1]	915
IBNR and Case Development Liabilities	678
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | Shortduration Insurance Contracts Accident Years 2011 Through 2020
Claims Development [Line Items]
Unpaid losses and ALAE, net	10,945
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | Shortduration Insurance Contracts Accident Years Before 2011
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	335
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	26,307
Cumulative Paid Losses and ALAE	12,295
Unpaid losses and ALAE, net	22,890
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,255	2,272	[1]	2,307		2,346		2,354		2,447		2,536		2,595		2,726		2,635
Cumulative Paid Losses and ALAE	1,776	1,748	[1]	1,713		1,673		1,595		1,501		1,412		1,169		824		$ 294
IBNR and Case Development Liabilities	279
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,581	2,588	[1]	2,645		2,712		2,827		2,899		2,837		3,002		2,820
Cumulative Paid Losses and ALAE	1,883	1,825	[1]	1,764		1,664		1,539		1,381		1,150		757		$ 312
IBNR and Case Development Liabilities	317
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	1,892	1,964	[1]	2,060		2,114		2,170		2,328		2,298		2,160
Cumulative Paid Losses and ALAE	1,273	1,215	[1]	1,155		1,052		947		818		530		$ 294
IBNR and Case Development Liabilities	387
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	1,970	1,980	[1]	1,944		2,030		2,068		2,099		1,900
Cumulative Paid Losses and ALAE	1,119	974	[1]	889		765		655		488		$ 153
IBNR and Case Development Liabilities	537
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	1,870	1,908	[1]	2,035		2,137		2,109		1,902
Cumulative Paid Losses and ALAE	1,029	938	[1]	846		725		500		$ 199
IBNR and Case Development Liabilities	455
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	1,922	2,003	[1]	2,047		2,138		1,928
Cumulative Paid Losses and ALAE	972	874	[1]	742		563		$ 255
IBNR and Case Development Liabilities	555
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,494	2,588	[1]	2,711		2,216
Cumulative Paid Losses and ALAE	1,282	830	[1]	574		$ 233
IBNR and Case Development Liabilities	762
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	3,931	3,455	[1]
Cumulative Paid Losses and ALAE	906	356	[1]
IBNR and Case Development Liabilities	1,924
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	3,883
Cumulative Paid Losses and ALAE	406
IBNR and Case Development Liabilities	2,754
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	3,509	3,585	[1]	2,948
Cumulative Paid Losses and ALAE	1,649	$ 875	[1]	$ 267
IBNR and Case Development Liabilities	1,183
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | Shortduration Insurance Contracts Accident Years 2011 Through 2020
Claims Development [Line Items]
Unpaid losses and ALAE, net	14,012
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | Shortduration Insurance Contracts Accident Years Before 2011
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	$ 8,878

[1] Unaudited required supplemental information