Investments in equity securities - Fair value by segment (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Summary of Investment Holdings [Line Items]
Investments in equity securities	$ 118,909	[1]	$ 117,505	[2]
Insurance and Other [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	116,432		115,464
Railroad, Utilities and Energy [Member] | Other Assets [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	1,494	[3]	1,103	[3]
Finance and Financial Products [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	$ 983		$ 938

[1] As of September 30, 2014, approximately 58% of the aggregate fair value was concentrated in the equity securities of four companies (American Express Company - $13.3 billion; Wells Fargo & Company - $25.1 billion; International Business Machines Corporation - $13.4 billion; and The Coca-Cola Company - $17.1 billion).

[2] As of December 31, 2013, approximately 55% of the aggregate fair value was concentrated in the equity securities of four companies (American Express Company - $13.8 billion; Wells Fargo & Company - $21.9 billion; International Business Machines Corporation - $12.8 billion; and The Coca-Cola Company - $16.5 billion).

[3] Included in other assets.