Unpaid losses and loss adjustment expenses - Losses and allocated loss adjustment expenses Incurred (Detail) - Insurance Group [Member] Claim in Thousands, $ in Millions	Dec. 31, 2024 USD ($) Claim	Dec. 31, 2023 USD ($)		Dec. 31, 2022 USD ($)	[1]	Dec. 31, 2021 USD ($)	[1]	Dec. 31, 2020 USD ($)	[1]	Dec. 31, 2019 USD ($)	[1]	Dec. 31, 2018 USD ($)	[1]	Dec. 31, 2017 USD ($)	[1]	Dec. 31, 2016 USD ($)	[1]	Dec. 31, 2015 USD ($)	[1]
Claims Development [Line Items]
Unpaid losses and ALAE, net	$ 104,671
GEICO [Member] | Physical Damage [Member]
Claims Development [Line Items]
Losses and ALAE incurred	22,995
Cumulative Losses and ALAE Paid	22,266
Unpaid losses and ALAE, net	885
GEICO [Member] | Physical Damage [Member] | 2023 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	11,900	$ 12,273
Cumulative Losses and ALAE Paid	11,771	11,567
IBNR and Case Development Liabilities	$ 111
Cumulative Number of Reported Claims | Claim	7,430
GEICO [Member] | Physical Damage [Member] | 2024 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 11,095
Cumulative Losses and ALAE Paid	10,495
IBNR and Case Development Liabilities	$ (454)
Cumulative Number of Reported Claims | Claim	6,380
GEICO [Member] | Physical Damage [Member] | 2023-2024 [Member]
Claims Development [Line Items]
Unpaid losses and ALAE, net	$ 729
GEICO [Member] | Physical Damage [Member] | Before 2023 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	156
GEICO [Member] | Auto Liability [Member]
Claims Development [Line Items]
Losses and ALAE incurred	84,467
Cumulative Losses and ALAE Paid	64,539
Unpaid losses and ALAE, net	20,818
GEICO [Member] | Auto Liability [Member] | 2020 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	13,462	13,593	[1]	$ 13,697		$ 14,024		$ 14,637
Cumulative Losses and ALAE Paid	12,946	12,608	[1]	11,794		9,839		5,395
IBNR and Case Development Liabilities	$ 382
Cumulative Number of Reported Claims | Claim	2,127
GEICO [Member] | Auto Liability [Member] | 2021 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 17,033	17,229	[1]	17,457		17,481
Cumulative Losses and ALAE Paid	15,796	14,863	[1]	12,681		6,450
IBNR and Case Development Liabilities	$ 907
Cumulative Number of Reported Claims | Claim	2,457
GEICO [Member] | Auto Liability [Member] | 2022 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 18,818	18,903	[1]	19,645
Cumulative Losses and ALAE Paid	16,031	13,838	[1]	7,614
IBNR and Case Development Liabilities	$ 2,065
Cumulative Number of Reported Claims | Claim	2,387
GEICO [Member] | Auto Liability [Member] | 2023 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 18,073	17,948	[1]
Cumulative Losses and ALAE Paid	12,682	7,191	[1]
IBNR and Case Development Liabilities	$ 3,864
Cumulative Number of Reported Claims | Claim	1,956
GEICO [Member] | Auto Liability [Member] | 2024 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 17,081
Cumulative Losses and ALAE Paid	7,084
IBNR and Case Development Liabilities	$ 4,730
Cumulative Number of Reported Claims | Claim	1,694
GEICO [Member] | Auto Liability [Member] | 2020-2024 [Member]
Claims Development [Line Items]
Unpaid losses and ALAE, net	$ 19,928
GEICO [Member] | Auto Liability [Member] | Before 2020 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	890
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member]
Claims Development [Line Items]
Losses and ALAE incurred	15,742
Cumulative Losses and ALAE Paid	6,802
Unpaid losses and ALAE, net	9,432
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2015 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	1,010	1,016	[1]	1,033		1,093		1,157		$ 1,218		$ 1,290		$ 1,269		$ 1,342		$ 1,374
Cumulative Losses and ALAE Paid	879	843	[1]	799		719		663		543		382		218		108		23
IBNR and Case Development Liabilities	$ 37
Cumulative Number of Reported Claims | Claim	12
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2016 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 1,172	1,188	[1]	1,216		1,288		1,341		1,394		1,414		1,416		1,392
Cumulative Losses and ALAE Paid	957	908	[1]	822		712		620		461		274		115		22
IBNR and Case Development Liabilities	$ 78
Cumulative Number of Reported Claims | Claim	15
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2017 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 1,310	1,315	[1]	1,349		1,382		1,474		1,495		1,499		1,466
Cumulative Losses and ALAE Paid	977	877	[1]	739		582		457		300		128		27
IBNR and Case Development Liabilities	$ 89
Cumulative Number of Reported Claims | Claim	21
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2018 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 1,583	1,606	[1]	1,616		1,580		1,659		1,650		1,602
Cumulative Losses and ALAE Paid	1,080	949	[1]	728		543		367		166		35
IBNR and Case Development Liabilities	$ 140
Cumulative Number of Reported Claims | Claim	24
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2019 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 1,579	1,534	[1]	1,614		1,663		1,691		1,670
Cumulative Losses and ALAE Paid	963	757	[1]	536		314		160		39
IBNR and Case Development Liabilities	$ 180
Cumulative Number of Reported Claims | Claim	22
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2020 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 1,606	1,631	[1]	1,698		1,751		1,704
Cumulative Losses and ALAE Paid	767	531	[1]	321		148		34
IBNR and Case Development Liabilities	$ 369
Cumulative Number of Reported Claims | Claim	33
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2021 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 1,714	1,787	[1]	1,855		1,852
Cumulative Losses and ALAE Paid	548	333	[1]	136		36
IBNR and Case Development Liabilities	$ 718
Cumulative Number of Reported Claims | Claim	27
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2022 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 1,846	1,912	[1]	1,927
Cumulative Losses and ALAE Paid	441	182	[1]	38
IBNR and Case Development Liabilities	$ 981
Cumulative Number of Reported Claims | Claim	25
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2023 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 1,918	1,964	[1]
Cumulative Losses and ALAE Paid	156	28	[1]
IBNR and Case Development Liabilities	$ 1,428
Cumulative Number of Reported Claims | Claim	27
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | 2024 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 2,004
Cumulative Losses and ALAE Paid	34
IBNR and Case Development Liabilities	$ 1,793
Cumulative Number of Reported Claims | Claim	22
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | Before 2015 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	$ 492
Berkshire Hathaway Primary Group [Member] | Medical Professional Liability [Member] | Short Duration Insurance Contracts Accident Years 2015 Through 2024 [Member]
Claims Development [Line Items]
Unpaid losses and ALAE, net	8,940
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member]
Claims Development [Line Items]
Losses and ALAE incurred	50,722
Cumulative Losses and ALAE Paid	27,115
Unpaid losses and ALAE, net	24,335
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2015 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	2,367	2,358	[1]	2,393		2,408		2,402		2,428		2,426		2,455		2,539		2,580
Cumulative Losses and ALAE Paid	2,095	2,070	[1]	2,041		1,966		1,873		1,766		1,507		1,187		804		329
IBNR and Case Development Liabilities	$ 178
Cumulative Number of Reported Claims | Claim	120
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2016 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 2,809	2,840	[1]	2,864		2,825		2,815		2,772		2,793		2,848		2,931
Cumulative Losses and ALAE Paid	2,420	2,377	[1]	2,303		2,140		1,998		1,765		1,359		908		373
IBNR and Case Development Liabilities	$ 277
Cumulative Number of Reported Claims | Claim	125
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2017 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 3,357	3,321	[1]	3,320		3,322		3,310		3,299		3,337		3,473
Cumulative Losses and ALAE Paid	2,784	2,673	[1]	2,492		2,279		2,050		1,645		1,133		480
IBNR and Case Development Liabilities	$ 395
Cumulative Number of Reported Claims | Claim	143
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2018 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 4,230	4,177	[1]	4,091		4,030		3,967		3,886		3,998
Cumulative Losses and ALAE Paid	3,396	3,120	[1]	2,746		2,324		1,902		1,340		583
IBNR and Case Development Liabilities	$ 570
Cumulative Number of Reported Claims | Claim	164
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2019 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 4,919	4,847	[1]	4,763		4,692		4,623		4,584
Cumulative Losses and ALAE Paid	3,812	3,430	[1]	2,898		2,214		1,598		725
IBNR and Case Development Liabilities	$ 691
Cumulative Number of Reported Claims | Claim	185
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2020 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 4,866	4,774	[1]	4,775		4,881		5,030
Cumulative Losses and ALAE Paid	3,138	2,598	[1]	2,066		1,498		736
IBNR and Case Development Liabilities	$ 1,167
Cumulative Number of Reported Claims | Claim	157
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2021 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 5,983	5,882	[1]	5,856		5,899
Cumulative Losses and ALAE Paid	3,168	2,440	[1]	1,751		869
IBNR and Case Development Liabilities	$ 1,923
Cumulative Number of Reported Claims | Claim	318
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2022 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 6,866	6,757	[1]	6,796
Cumulative Losses and ALAE Paid	2,905	2,012	[1]	962
IBNR and Case Development Liabilities	$ 2,801
Cumulative Number of Reported Claims | Claim	316
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2023 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 7,363	7,299	[1]
Cumulative Losses and ALAE Paid	2,152	1,056	[1]
IBNR and Case Development Liabilities	$ 3,887
Cumulative Number of Reported Claims | Claim	187
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | 2024 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	$ 7,962
Cumulative Losses and ALAE Paid	1,245
IBNR and Case Development Liabilities	$ 5,552
Cumulative Number of Reported Claims | Claim	169
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | Before 2015 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	$ 728
Berkshire Hathaway Primary Group [Member] | Workers' Compensation and Other Casualty [Member] | Short Duration Insurance Contracts Accident Years 2015 Through 2024 [Member]
Claims Development [Line Items]
Unpaid losses and ALAE, net	23,607
Berkshire Hathaway Reinsurance Group [Member] | Property [Member]
Claims Development [Line Items]
Losses and ALAE incurred	59,362
Cumulative Losses and ALAE Paid	43,998
Unpaid losses and ALAE, net	15,851
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2015 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	3,225	3,255	[1]	3,232		3,238		3,236		3,214		3,218		2,827		3,392		3,587
Cumulative Losses and ALAE Paid	2,966	2,917	[1]	2,843		2,791		2,705		2,522		2,414		2,196		1,793		669
IBNR and Case Development Liabilities	94
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2016 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	4,091	4,088	[1]	4,099		4,121		4,132		4,134		4,170		4,492		3,905
Cumulative Losses and ALAE Paid	3,889	3,802	[1]	3,724		3,615		3,419		3,151		2,662		2,203		913
IBNR and Case Development Liabilities	30
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2017 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	5,577	5,590	[1]	5,608		5,689		5,767		5,893		6,071		6,374
Cumulative Losses and ALAE Paid	5,456	5,432	[1]	5,343		5,150		4,915		4,564		3,494		1,346
IBNR and Case Development Liabilities	51
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2018 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	5,238	5,218	[1]	5,267		5,260		5,402		5,542		5,456
Cumulative Losses and ALAE Paid	4,571	4,444	[1]	4,236		3,992		3,709		3,073		1,204
IBNR and Case Development Liabilities	207
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2019 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	4,447	4,488	[1]	4,537		4,838		5,064		4,951
Cumulative Losses and ALAE Paid	4,082	4,018	[1]	3,856		3,532		2,826		1,009
IBNR and Case Development Liabilities	112
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2020 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	6,450	6,620	[1]	6,825		7,188		6,914
Cumulative Losses and ALAE Paid	5,546	5,275	[1]	4,613		3,569		1,252
IBNR and Case Development Liabilities	376
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2021 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	7,289	7,585	[1]	7,919		8,063
Cumulative Losses and ALAE Paid	6,073	5,377	[1]	4,070		1,621
IBNR and Case Development Liabilities	450
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2022 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	7,916	8,412	[1]	8,852
Cumulative Losses and ALAE Paid	5,588	4,322	[1]	1,813
IBNR and Case Development Liabilities	1,102
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2023 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	7,379	7,953	[1]
Cumulative Losses and ALAE Paid	4,107	1,771	[1]
IBNR and Case Development Liabilities	1,632
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2024 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	7,750
Cumulative Losses and ALAE Paid	1,720
IBNR and Case Development Liabilities	4,225
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | Before 2015 [Member]
Claims Development [Line Items]
Unpaid losses and ALAE, net	487
Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | Short Duration Insurance Contracts Accident Years 2015 Through 2024 [Member]
Claims Development [Line Items]
Unpaid losses and ALAE, net	15,364
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member]
Claims Development [Line Items]
Losses and ALAE incurred	52,757
Cumulative Losses and ALAE Paid	28,849
Unpaid losses and ALAE, net	33,350
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2015 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	3,276	3,300	[1]	3,298		3,351		3,388		3,426		3,568		3,655		3,615		3,409
Cumulative Losses and ALAE Paid	2,678	2,582	[1]	2,473		2,338		2,190		1,996		1,724		1,425		990		$ 445
IBNR and Case Development Liabilities	311
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2016 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	3,793	3,763	[1]	3,735		3,765		3,808		3,859		3,891		3,987		3,777
Cumulative Losses and ALAE Paid	3,095	2,957	[1]	2,782		2,607		2,414		2,143		1,795		1,355		$ 653
IBNR and Case Development Liabilities	370
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2017 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	4,214	4,211	[1]	4,154		4,164		4,232		4,339		4,462		4,028
Cumulative Losses and ALAE Paid	3,350	3,164	[1]	2,938		2,720		2,472		1,823		1,306		$ 605
IBNR and Case Development Liabilities	432
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2018 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	5,429	5,359	[1]	5,273		5,345		5,476		5,523		4,875
Cumulative Losses and ALAE Paid	4,224	3,948	[1]	3,631		3,252		2,786		1,737		$ 693
IBNR and Case Development Liabilities	647
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2019 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	5,751	5,678	[1]	5,718		5,885		6,025		5,559
Cumulative Losses and ALAE Paid	4,094	3,727	[1]	3,181		2,414		1,865		$ 839
IBNR and Case Development Liabilities	1,002
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2020 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	6,053	6,079	[1]	6,033		6,215		6,203
Cumulative Losses and ALAE Paid	3,967	3,373	[1]	2,709		1,900		$ 866
IBNR and Case Development Liabilities	1,371
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2021 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	5,959	5,942	[1]	6,270		6,300
Cumulative Losses and ALAE Paid	3,225	2,654	[1]	1,781		$ 765
IBNR and Case Development Liabilities	1,838
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2022 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	6,045	6,092	[1]	6,047
Cumulative Losses and ALAE Paid	1,963	1,333	[1]	$ 620
IBNR and Case Development Liabilities	2,829
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2023 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	6,144	6,053	[1]
Cumulative Losses and ALAE Paid	1,575	$ 689	[1]
IBNR and Case Development Liabilities	3,528
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2024 [Member]
Claims Development [Line Items]
Losses and ALAE incurred	6,093
Cumulative Losses and ALAE Paid	678
IBNR and Case Development Liabilities	4,660
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | Before 2015 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	9,442
Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | Short Duration Insurance Contracts Accident Years 2015 Through 2024 [Member]
Claims Development [Line Items]
Unpaid losses and ALAE, net	$ 23,908

[1] Unaudited required supplemental information