Investments in equity securities (Detail) - USD ($) $ in Millions	Mar. 31, 2018		Dec. 31, 2017
Summary of Investment Holdings [Line Items]
Cost basis of investments	$ 85,830		$ 74,676
Net Unrealized Holding Gains on investments	87,146		95,864
Fair value of investments	172,976	[1]	170,540	[2]
Banks, insurance and finance [Member]
Summary of Investment Holdings [Line Items]
Cost basis of investments	25,986		25,783
Net Unrealized Holding Gains on investments	50,091		55,026
Fair value of investments	76,077		80,809
Consumer products [Member]
Summary of Investment Holdings [Line Items]
Cost basis of investments	38,130		25,177
Net Unrealized Holding Gains on investments	23,864		25,698
Fair value of investments	61,994		50,875
Commercial, industrial and other [Member]
Summary of Investment Holdings [Line Items]
Cost basis of investments	21,714		23,716
Net Unrealized Holding Gains on investments	13,191		15,140
Fair value of investments	$ 34,905		$ 38,856

[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).