Investments in equity securities (Detail) - USD ($) $ in Millions	Jun. 30, 2018		Dec. 31, 2017
Summary of Investment Holdings [Line Items]
Cost Basis	$ 87,458		$ 74,676
Net Unrealized Gains	92,271		95,864
Fair Value	179,729	[1]	170,540	[2]
Banks, insurance and finance [Member]
Summary of Investment Holdings [Line Items]
Cost Basis	27,003		25,783
Net Unrealized Gains	50,770		55,026
Fair Value	77,773		80,809
Consumer products [Member]
Summary of Investment Holdings [Line Items]
Cost Basis	40,199		25,177
Net Unrealized Gains	28,502		25,698
Fair Value	68,701		50,875
Commercial, industrial and other [Member]
Summary of Investment Holdings [Line Items]
Cost Basis	20,256		23,716
Net Unrealized Gains	12,999		15,140
Fair Value	$ 33,255		$ 38,856

[1] Approximately 70% of the aggregate fair value was concentrated in five companies (American Express Company - $14.9 billion; Apple Inc. - $47.2 billion; Bank of America Corporation - $19.7 billion; The Coca-Cola Company - $17.5 billion and Wells Fargo & Company - $26.4 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).