Investments in equity securities - Fair value by segment (Detail) - USD ($) $ in Millions		Sep. 30, 2016		Dec. 31, 2015
Summary of Investment Holdings [Line Items]
Investments in equity securities		$ 102,535	[1]	$ 111,822	[2]
Insurance and Other [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities		100,757		110,212
Railroad, Utilities and Energy [Member] | Other Assets [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	[3]	1,476		1,238
Finance and Financial Products [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities		$ 302		$ 372

[1] Approximately 60% of the aggregate fair value was concentrated in the equity securities of four companies (American Express Company - $9.7 billion; Wells Fargo & Company - $22.1 billion; International Business Machines Corporation ("IBM") - $12.9 billion; and The Coca-Cola Company - $16.9 billion).

[2] Approximately 59% of the aggregate fair value was concentrated in the equity securities of four companies (American Express Company - $10.5 billion; Wells Fargo & Company - $27.2 billion; IBM - $11.2 billion; and The Coca-Cola Company - $17.2 billion).

[3] Included in other assets.