Investments in equity securities - Fair value by segment (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Summary of Investment Holdings [Line Items]
Investments in equity securities	$ 115,531	[1]	$ 117,470	[2]
Insurance and Other [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	113,341		115,529
Railroad, Utilities and Energy [Member] | Other Assets [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	1,158	[3]	881	[3]
Finance and Financial Products [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	$ 1,032		$ 1,060

[1] Approximately 58% of the aggregate fair value was concentrated in the equity securities of four companies (American Express Company - $11.8 billion; Wells Fargo & Company - $26.7 billion; International Business Machines Corporation - $12.8 billion; and The Coca-Cola Company - $16.2 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 - $12.3 billion; and The Coca-Cola Company - $16.9 billion).

[3] Included in other assets.