Investments in equity securities - Fair value by segment (Detail) - USD ($) $ in Millions		Dec. 31, 2015		Dec. 31, 2014
Summary of Investment Holdings [Line Items]
Investments in equity securities		$ 111,822	[1]	$ 117,470	[2]
Insurance and Other [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities		110,212		115,529
Railroad, Utilities and Energy [Member] | Other Assets [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	[3]	1,238		881
Finance and Financial Products [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities		$ 372		$ 1,060

[1] 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 ("IBM")-$11.2 billion; and The Coca-Cola Company-$17.2 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.