Investments in equity securities - Fair value by segment (Detail) - USD ($) $ in Millions		Sep. 30, 2015		Dec. 31, 2014
Summary of Investment Holdings [Line Items]
Investments in equity securities		$ 110,288	[1]	$ 117,470	[2]
Insurance and Other [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities		106,104		115,529
Railroad, Utilities and Energy [Member] | Other Assets [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	[3]	1,185		881
Finance and Financial Products [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities		$ 2,999		$ 1,060

[1] Approximately 58% of the aggregate fair value was concentrated in the equity securities of four companies (American Express Company - $11.2 billion; Wells Fargo & Company - $25.2 billion; International Business Machines Corporation ("IBM") - $11.7 billion; and The Coca-Cola Company - $16.0 billion).

[2] Approximately 59% of the aggregate fair value was concentrated in the equity securities of four companies (American Express Company - $14.1 billion; Wells Fargo & Company - $26.5 billion; International Business Machines Corporation ("IBM") - $12.3 billion; and The Coca-Cola Company - $16.9 billion).

[3] Included in other assets.