Investments in equity securities - Fair value by segment (Detail) - USD ($) $ in Millions		Mar. 31, 2016		Dec. 31, 2015
Summary of Investment Holdings [Line Items]
Investments in equity securities		$ 106,420	[1]	$ 111,822	[2]
Insurance and Other [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities		104,798		110,212
Railroad, Utilities and Energy [Member] | Other Assets [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	[3]	1,291		1,238
Finance and Financial Products [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities		$ 331		$ 372

[1] Approximately 60% of the aggregate fair value was concentrated in the equity securities of four companies (American Express Company - $9.3 billion; Wells Fargo & Company - $24.2 billion; International Business Machines Corporation ("IBM") - $12.3 billion; and The Coca-Cola Company - $18.6 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; International Business Machines Corporation - $11.2 billion; and The Coca-Cola Company - $17.2 billion).

[3] Included in other assets.