Investments in equity securities - Fair value by segment (Detail) - USD ($) $ in Millions		Sep. 30, 2018		Dec. 31, 2017
Summary of Investment Holdings [Line Items]
Investments in equity securities		$ 207,332	[1]	$ 170,540	[2]
Insurance and Other [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities		201,226		164,026
Railroad, Utilities and Energy [Member] | Other Assets [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	[3]	1,616		1,961
Finance and Financial Products [Member] | Other Assets [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	[3]	$ 4,490		$ 4,553

[1] Approximately 69% of the aggregate fair value was concentrated in five companies (American Express Company - $16.1 billion; Apple Inc. - $57.6 billion; Bank of America Corporation - $26.5 billion; The Coca-Cola Company - $18.5 billion and Wells Fargo & Company - $24.4 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.