Investment gains/losses - Narrative (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Bank of America Corporation [Member] | Non-Cumulative Perpetual Preferred Stock [Member]
Gain (Loss) on Investments [Line Items]
Gross gains	$ 1,000
Wm Wrigley Jr Company, The Kraft Heinz Company, and Dow Chemical Company [Member] | Preferred Stock and Common Stock [Member]
Gain (Loss) on Investments [Line Items]
Gross gains		$ 4,200
Duracell Company [Member] | Procter & Gamble Company [Member] | Common Stock [Member]
Gain (Loss) on Investments [Line Items]
Gross gains		$ 1,100
Berkshire Hathaway Inc. (Parent) [Member]
Gain (Loss) on Investments [Line Items]
Non-cash pre-tax holding gain			$ 6,838
Berkshire Hathaway Inc. (Parent) [Member] | Kraft Heinz (previously Heinz Holding) [Member] | Common Stock [Member]
Gain (Loss) on Investments [Line Items]
Non-cash pre-tax holding gain			$ 6,838