Investments in equity securities - Fair value by segment (Detail) - USD ($) $ in Millions		Dec. 31, 2017		Dec. 31, 2016
Summary of Investment Holdings [Line Items]
Investments in equity securities		$ 170,540	[1]	$ 139,288	[2]
Insurance and Other [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities		164,026		134,835
Railroad, Utilities and Energy [Member] | Other Assets [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	[3]	1,961		1,186
Finance and Financial Products [Member] | Other Assets [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	[3]	$ 4,553		$ 3,267

[1] Approximately 65% of the aggregate fair value was concentrated in five companies (American Express Company - $15.1 billion; Apple Inc. - $28.2 billion; Bank of America Corporation - $20.7 billion; The Coca-Cola Company - $18.4 billion and Wells Fargo & Company - $29.3 billion).

[2] Approximately 60% of the aggregate fair value was concentrated in five companies (American Express Company - $11.2 billion; Bank of America Corporation - $14.5 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.