Common stock - Narrative (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Vote shares	Dec. 31, 2016 shares	Dec. 31, 2015 shares	Dec. 31, 2014 shares
Class of Stock [Line Items]
Preferred Stock, shares authorized	1,000,000
Preferred Stock, shares issued	0
Class B [Member]
Class of Stock [Line Items]
Ratio of net earnings per equivalent common share and dividend and distribution rights of Class B shares to Class A shares	0.000667
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	1,340,656,987	1,301,914,165	1,252,456,836	1,224,855,488
Class A [Member]
Class of Stock [Line Items]
Number of votes entitled per share, number | Vote	1
Shares outstanding	751,075	776,378	808,422	826,339
Equivalent Class A [Member]
Class of Stock [Line Items]
Shares outstanding	1,644,846	1,644,321
Common Stock Repurchase Program [Member]
Class of Stock [Line Items]
Shares repurchase, authorization description	Berkshire's Board of Directors has approved a common stock repurchase program permitting Berkshire to repurchase its Class A and Class B shares at prices no higher than a 20% premium over the book value of the shares
Minimum cash, cash equivalents and U.S. Treasury Bills threshold after repurchase of common stock shares, amount | $	$ 20,000,000,000