Investments in equity securities - Narrative (Detail) - Investment Commitment [Member] - USD ($) $ / shares in Units, $ in Billions			9 Months Ended
	Aug. 03, 2020	Aug. 08, 2019	Sep. 30, 2020
Summary of Investment Holdings [Line Items]
Investment commitment description			On August 8, 2019, Berkshire invested a total of $10 billion in Occidental Corporation (“Occidental”) newly issued Occidental Cumulative Perpetual Preferred Stock with an aggregate liquidation value of $10 billion, and warrants to purchase up to 80 million shares of Occidental common stock at an exercise price of $62.50 per share. In accordance with the terms of the warrants, on August 3, 2020, the number of shares of common stock that can be purchased was increased to 83.86 million shares and the exercise price was reduced to $59.62 per share. The preferred stock accrues dividends at 8% per annum and is redeemable at the option of Occidental commencing in 2029 at a redemption price equal to 105% of the liquidation preference plus any accumulated and unpaid dividends, or mandatorily under certain specified capital return events. Dividends on the preferred stock may be paid in cash or, at Occidental’s option, in shares of Occidental common stock. The warrants are exercisable in whole or in part until one year after the redemption of the preferred stock. Our investments in Occidental are included in the commercial, industrial and other category in the preceding tables.
Occidental Corporation [Member] | Cumulative Perpetual Preferred Stock and Warrants [Member]
Summary of Investment Holdings [Line Items]
Investment commitment amount		$ 10
Occidental Corporation [Member] | Cumulative Perpetual Preferred Stock [Member]
Summary of Investment Holdings [Line Items]
Investment in preferred stock, liquidation value		$ 10
Investment in preferred stock, stated dividend rate		8.00%
Investment in preferred stock, redemption year start		2029
Investment in preferred stock, redemption price percentage of liquidation preference		105.00%
Occidental Corporation [Member] | Warrant [Member]
Summary of Investment Holdings [Line Items]
Number of common shares that can be purchased	83,860,000	80,000,000
Exercise price for warrants, per share		$ 62.50
Exercise price for warrants, per share	$ 59.62