Investments in equity securities - Fair value by segment (Detail) - USD ($) $ in Millions		Mar. 31, 2018		Dec. 31, 2017
Summary of Investment Holdings [Line Items]
Investments in equity securities		$ 172,976	[1]	$ 170,540	[2]
Insurance and Other [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities		166,658		164,026
Railroad, Utilities and Energy [Member] | Other Assets [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	[3]	1,755		1,961
Finance and Financial Products [Member] | Other Assets [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	[3]	$ 4,563		$ 4,553

[1] Approximately 68% of the aggregate fair value was concentrated in five companies (American Express Company - $14.1 billion; Apple Inc. - $40.7 billion; Bank of America Corporation - $21.0 billion; The Coca-Cola Company - $17.4 billion and Wells Fargo & Company - $25.2 billion).

[2] 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).

[3] Included in other assets.