Common stock - Narrative (Detail)	9 Months Ended
	Sep. 30, 2020 USD ($) Vote shares	Dec. 31, 2019 shares
Class of Stock [Line Items]
Preferred Stock, shares authorized	1,000,000
Preferred Stock, shares issued	0
Economic equivalent of Class B share to Class A share	0.0667%
Common Stock Repurchase Program [Member]
Class of Stock [Line Items]
Shares repurchase, authorization description	Berkshire’s common stock repurchase program permits Berkshire to repurchase its Class A and Class B shares any time that Warren Buffett, Berkshire’s Chairman of the Board and Chief Executive Officer, and Charlie Munger, Vice Chairman of the Board, believe that the repurchase price is below Berkshire’s intrinsic value, conservatively determined.
Minimum cash, cash equivalents and U.S. Treasury Bills threshold after repurchase of common stock shares, amount | $	$ 20,000,000,000
Class A [Member]
Class of Stock [Line Items]
Number of votes entitled per share, number | Vote	1
Shares outstanding	650,946	701,970
Class B [Member]
Class of Stock [Line Items]
Number of votes entitled per share, number | Vote	0.0001
Number of shares of Class B stock obtainable from converting one Class A share	1,500
Shares outstanding	1,379,535,071	1,384,481,533
Equivalent Class A [Member]
Class of Stock [Line Items]
Shares outstanding	1,570,636	1,624,958