Investments in equity securities - Fair value by segment (Detail) - USD ($) $ in Millions		Sep. 30, 2017		Dec. 31, 2016
Summary of Investment Holdings [Line Items]
Investments in equity securities		$ 157,650	[1]	$ 122,032	[2]
Insurance and Other [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities		151,639		120,471
Railroad, Utilities and Energy [Member] | Other Assets [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	[3]	2,087		1,186
Finance and Financial Products [Member] | Other Assets [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	[3]	$ 3,924		$ 375

[1] Approximately 62% of the aggregate fair value was concentrated in the equity securities of five companies: American Express Company - $13.7 billion, Apple Inc. - $21.3 billion, Bank of America Corporation - $17.7 billion, The Coca-Cola Company - $18.0 billion and Wells Fargo & Company - $26.9 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, International Business Machines Corporation - $13.5 billion and Wells Fargo & Company - $27.6 billion.

[3] Included in other assets.