Investments in equity securities (Detail) - USD ($) $ in Millions	Dec. 31, 2017		Dec. 31, 2016
Summary of Investment Holdings [Line Items]
Cost basis of investments	$ 76,202		$ 74,097
Unrealized gains on investments	94,548		65,901
Unrealized losses on investments	(210)		(710)
Fair value of investments	170,540	[1]	139,288	[2]
Banks, insurance and finance [Member]
Summary of Investment Holdings [Line Items]
Cost basis of investments	27,318		19,852
Unrealized gains on investments	53,491		30,572
Fair value of investments	80,809		50,424
Consumer products [Member]
Summary of Investment Holdings [Line Items]
Cost basis of investments	24,855		10,657
Unrealized gains on investments	26,088		16,760
Unrealized losses on investments	(68)		(9)
Fair value of investments	50,875		27,408
Commercial, industrial and other [Member]
Summary of Investment Holdings [Line Items]
Cost basis of investments	24,029		35,868
Unrealized gains on investments	14,969		9,033
Unrealized losses on investments	(142)		(701)
Fair value of investments	$ 38,856		44,200
Other Investments [Member]
Summary of Investment Holdings [Line Items]
Cost basis of investments			7,720
Unrealized gains on investments			9,536
Fair value of investments			$ 17,256

[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).