Investments in equity securities (Detail) - USD ($) $ in Millions	Sep. 30, 2021		Dec. 31, 2020
Summary of Investment Holdings [Line Items]
Cost Basis	$ 104,188		$ 108,620
Net Unrealized Gains	206,551		172,550
Fair Value	310,739	[1]	281,170	[2]
Banks, insurance and finance [Member]
Summary of Investment Holdings [Line Items]
Cost Basis	26,809		26,312
Net Unrealized Gains	60,931		40,167
Fair Value	87,740		66,479
Consumer products [Member]
Summary of Investment Holdings [Line Items]
Cost Basis	34,973		34,747
Net Unrealized Gains	119,421		111,583
Fair Value	154,394		146,330
Commercial, industrial and other [Member]
Summary of Investment Holdings [Line Items]
Cost Basis	42,406		47,561
Net Unrealized Gains	26,199		20,800
Fair Value	$ 68,605		$ 68,361

[1] Approximately 70% of the aggregate fair value was concentrated in four companies (American Express Company – $25.4 billion; Apple Inc. – $128.4 billion; Bank of America Corporation – $43.9 billion and The Coca-Cola Company – $21.0 billion).

[2] Approximately 68% of the aggregate fair value was concentrated in four companies (American Express Company – $18.3 billion; Apple Inc. – $120.4 billion; Bank of America Corporation – $31.3 billion and The Coca-Cola Company – $21.9 billion).