Investments in equity securities - Fair value by segment (Detail) - USD ($) $ in Millions		Mar. 31, 2017		Dec. 31, 2016
Summary of Investment Holdings [Line Items]
Investments in equity securities		$ 135,020	[1]	$ 122,032	[2]
Insurance and Other [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities		133,393		120,471
Railroad, Utilities and Energy [Member] | Other Assets [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	[3]	1,248		1,186
Finance and Financial Products [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities		$ 379		$ 375

[1] Approximately 65% of the aggregate fair value was concentrated in the equity securities of five companies: American Express Company - $12.0 billion, Apple Inc. - $19.2 billion, The Coca-Cola Company - $17.0 billion, International Business Machines Corporation ("IBM") - $11.2 billion and Wells Fargo & Company - $27.8 billion.

[2] Approximately 62% of the aggregate fair value was concentrated in the equity securities of five companies: American Express Company - $11.2 billion, Apple Inc. - $7.1 billion, The Coca-Cola Company - $16.6 billion, IBM - $13.5 billion and Wells Fargo & Company - $27.6 billion.

[3] Included in other assets.