Unpaid losses and loss adjustment expenses - Incurred and paid losses and allocated loss adjustment expenses (Detail) Claim in Thousands, $ in Millions	Dec. 31, 2017 USD ($) Claim	Dec. 31, 2016 USD ($)	[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 ($)	[1]	Dec. 31, 2008 USD ($)	[1]
Claims Development [Line Items]
Net unpaid losses and ALAE	$ 55,579
Insurance Group [Member] | GEICO [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	86,608
Cumulative Paid Losses and ALAE	71,493
Net unpaid losses and ALAE	15,655
Insurance Group [Member] | GEICO [Member] | 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	12,961	$ 12,920		$ 12,943		$ 12,900		$ 13,085
IBNR and Case Development Liabilities	$ 105
Cumulative Number of Reported Claims | Claim	7,105
Cumulative Paid Losses and ALAE	$ 12,618	12,256		11,650		10,573		8,006
Insurance Group [Member] | GEICO [Member] | 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	14,589	14,559		14,572		14,680
IBNR and Case Development Liabilities	$ 236
Cumulative Number of Reported Claims | Claim	7,972
Cumulative Paid Losses and ALAE	$ 13,850	13,140		12,036		9,199
Insurance Group [Member] | GEICO [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	16,993	16,875		16,887
IBNR and Case Development Liabilities	$ 682
Cumulative Number of Reported Claims | Claim	8,915
Cumulative Paid Losses and ALAE	$ 15,285	13,858		10,606
Insurance Group [Member] | GEICO [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	19,390	19,106
IBNR and Case Development Liabilities	$ 1,806
Cumulative Number of Reported Claims | Claim	9,601
Cumulative Paid Losses and ALAE	$ 15,862	12,020
Insurance Group [Member] | GEICO [Member] | 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	22,675
IBNR and Case Development Liabilities	$ 4,343
Cumulative Number of Reported Claims | Claim	10,513
Cumulative Paid Losses and ALAE	$ 13,878
Insurance Group [Member] | GEICO [Member] | 2013-2017 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE	15,115
Insurance Group [Member] | GEICO [Member] | Before 2013 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	540
Insurance Group [Member] | Berkshire Hathaway Primary Group [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	22,584
Cumulative Paid Losses and ALAE	11,149
Net unpaid losses and ALAE	11,867
Insurance Group [Member] | Berkshire Hathaway Primary Group [Member] | 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	1,900	1,977		2,052		2,127		2,199
IBNR and Case Development Liabilities	$ 424
Cumulative Number of Reported Claims | Claim	100
Cumulative Paid Losses and ALAE	$ 1,272	1,118		903		621		279
Insurance Group [Member] | Berkshire Hathaway Primary Group [Member] | 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,580	2,687		2,737		2,906
IBNR and Case Development Liabilities	$ 736
Cumulative Number of Reported Claims | Claim	138
Cumulative Paid Losses and ALAE	$ 1,504	1,192		833		387
Insurance Group [Member] | Berkshire Hathaway Primary Group [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	3,266	3,406		3,519
IBNR and Case Development Liabilities	$ 1,095
Cumulative Number of Reported Claims | Claim	163
Cumulative Paid Losses and ALAE	$ 1,498	1,060		499
Insurance Group [Member] | Berkshire Hathaway Primary Group [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	4,024	4,149
IBNR and Case Development Liabilities	$ 1,917
Cumulative Number of Reported Claims | Claim	171
Cumulative Paid Losses and ALAE	$ 1,302	634
Insurance Group [Member] | Berkshire Hathaway Primary Group [Member] | 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	5,024
IBNR and Case Development Liabilities	$ 3,161
Cumulative Number of Reported Claims | Claim	166
Cumulative Paid Losses and ALAE	$ 761
Insurance Group [Member] | Berkshire Hathaway Primary Group [Member] | 2008 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	1,063	1,073		1,103		1,135		1,182		$ 1,250		$ 1,369		$ 1,448		$ 1,503		$ 1,573
IBNR and Case Development Liabilities	$ 69
Cumulative Number of Reported Claims | Claim	71
Cumulative Paid Losses and ALAE	$ 968	950		932		903		852		771		676		535		369		181
Insurance Group [Member] | Berkshire Hathaway Primary Group [Member] | 2009 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	1,047	1,073		1,109		1,164		1,229		1,322		1,392		1,435		1,528
IBNR and Case Development Liabilities	$ 75
Cumulative Number of Reported Claims | Claim	60
Cumulative Paid Losses and ALAE	$ 933	921		891		838		752		643		507		335		136
Insurance Group [Member] | Berkshire Hathaway Primary Group [Member] | 2010 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	1,068	1,119		1,163		1,234		1,321		1,378		1,437		1,516
IBNR and Case Development Liabilities	$ 107
Cumulative Number of Reported Claims | Claim	61
Cumulative Paid Losses and ALAE	$ 916	889		846		768		661		522		366		153
Insurance Group [Member] | Berkshire Hathaway Primary Group [Member] | 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	1,189	1,249		1,290		1,359		1,446		1,461		1,563
IBNR and Case Development Liabilities	$ 168
Cumulative Number of Reported Claims | Claim	67
Cumulative Paid Losses and ALAE	$ 946	903		824		682		533		331		167
Insurance Group [Member] | Berkshire Hathaway Primary Group [Member] | 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	1,423	1,518		1,559		1,631		1,675		1,710
IBNR and Case Development Liabilities	$ 250
Cumulative Number of Reported Claims | Claim	79
Cumulative Paid Losses and ALAE	$ 1,049	956		820		642		444		165
Insurance Group [Member] | Berkshire Hathaway Primary Group [Member] | 2008-2017 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE	11,435
Insurance Group [Member] | Berkshire Hathaway Primary Group [Member] | Before 2008 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	432
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Property [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	29,099
Cumulative Paid Losses and ALAE	20,691
Net unpaid losses and ALAE	8,838
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,614	2,636		2,727		3,074		3,230
IBNR and Case Development Liabilities	172
Cumulative Paid Losses and ALAE	2,211	2,090		1,892		1,447		522
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,199	2,344		2,458		2,646
IBNR and Case Development Liabilities	223
Cumulative Paid Losses and ALAE	1,731	1,591		1,259		467
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,557	3,115		3,268
IBNR and Case Development Liabilities	232
Cumulative Paid Losses and ALAE	1,959	1,604		571
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	3,938	3,294
IBNR and Case Development Liabilities	1,072
Cumulative Paid Losses and ALAE	1,807	706
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	5,276
IBNR and Case Development Liabilities	1,993
Cumulative Paid Losses and ALAE	1,028
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2008 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,336	2,344		2,353		2,386		2,432		2,392		2,483		2,773		2,720		3,030
IBNR and Case Development Liabilities	6
Cumulative Paid Losses and ALAE	2,308	2,307		2,296		2,240		2,205		2,159		2,055		1,825		1,449		481
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2009 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,000	2,001		2,006		2,048		2,075		2,171		2,250		2,057		2,385
IBNR and Case Development Liabilities	8
Cumulative Paid Losses and ALAE	1,977	1,975		1,965		1,931		1,861		1,792		1,627		1,122		405
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2010 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,126	2,146		2,164		2,200		2,289		2,424		2,538		2,545
IBNR and Case Development Liabilities	32
Cumulative Paid Losses and ALAE	2,079	2,054		2,007		1,950		1,789		1,520		1,081		339
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	3,706	3,729		3,761		3,764		3,850		4,127		4,192
IBNR and Case Development Liabilities	84
Cumulative Paid Losses and ALAE	3,475	3,433		3,344		3,233		2,963		2,308		661
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,347	2,351		2,404		2,645		2,851		3,153
IBNR and Case Development Liabilities	95
Cumulative Paid Losses and ALAE	2,116	2,041		1,952		1,813		1,232		265
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | 2008-2017 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE	8,408
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Property [Member] | Before 2008 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	430
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	21,770
Cumulative Paid Losses and ALAE	11,284
Net unpaid losses and ALAE	19,219
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,107	2,162		2,320		2,290		2,152
IBNR and Case Development Liabilities	644
Cumulative Paid Losses and ALAE	1,048	943		814		527		293
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,021	2,059		2,090		1,891
IBNR and Case Development Liabilities	736
Cumulative Paid Losses and ALAE	761	651		485		152
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,130	2,102		1,895
IBNR and Case Development Liabilities	803
Cumulative Paid Losses and ALAE	722	497		198
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,132	1,923
IBNR and Case Development Liabilities	1,012
Cumulative Paid Losses and ALAE	561	254
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,209
IBNR and Case Development Liabilities	1,428
Cumulative Paid Losses and ALAE	232
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2008 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,010	2,009		2,042		2,101		2,161		2,309		2,358		2,279		2,602		2,448
IBNR and Case Development Liabilities	226
Cumulative Paid Losses and ALAE	1,442	1,405		1,367		1,325		1,250		1,168		1,067		959		664		$ 253
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2009 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,213	2,252		2,315		2,362		2,437		2,500		2,571		2,711		2,393
IBNR and Case Development Liabilities	225
Cumulative Paid Losses and ALAE	1,695	1,669		1,632		1,596		1,439		1,219		947		858		$ 249
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2010 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	1,903	2,066		2,111		2,162		2,282		2,345		2,413		2,320
IBNR and Case Development Liabilities	166
Cumulative Paid Losses and ALAE	1,493	1,448		1,384		1,288		1,035		846		553		$ 120
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,340	2,348		2,440		2,529		2,589		2,720		2,628
IBNR and Case Development Liabilities	419
Cumulative Paid Losses and ALAE	1,670	1,591		1,498		1,409		1,167		822		$ 293
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE	2,705	2,819		2,892		2,829		2,995		2,811
IBNR and Case Development Liabilities	663
Cumulative Paid Losses and ALAE	1,660	$ 1,535		$ 1,378		$ 1,147		$ 754		$ 311
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | 2008-2017 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE	10,486
Insurance Group [Member] | Berkshire Hathaway Reinsurance Group [Member] | Casualty [Member] | Before 2008 [Member]
Claims Development [Line Items]
Net unpaid losses and ALAE for accident years before earliest accident year presented	$ 8,733

[1] Unaudited supplemental information