Investments in equity securities (Detail) - USD ($) $ in Millions	Sep. 30, 2020		Dec. 31, 2019
Summary of Investment Holdings [Line Items]
Cost Basis	$ 105,629		$ 110,340
Net Unrealized Gains	139,688		137,687
Fair Value	245,317	[1]	248,027	[2]
Banks, insurance and finance [Member]
Summary of Investment Holdings [Line Items]
Cost Basis	27,145		40,419
Net Unrealized Gains	29,065		61,976
Fair Value	56,210		102,395
Consumer products [Member]
Summary of Investment Holdings [Line Items]
Cost Basis	37,016		38,887
Net Unrealized Gains	98,022		60,747
Fair Value	135,038		99,634
Commercial, industrial and other [Member]
Summary of Investment Holdings [Line Items]
Cost Basis	41,468		31,034
Net Unrealized Gains	12,601		14,964
Fair Value	$ 54,069		$ 45,998

[1] Approximately 70% of the aggregate fair value was concentrated in four companies (American Express Company – $15.2 billion; Apple Inc. – $111.7 billion; Bank of America Corporation – $24.9 billion and The Coca-Cola Company – $19.7 billion).

[2] Approximately 60% of the aggregate fair value was concentrated in four companies (American Express Company – $18.9 billion; Apple Inc. – $73.7 billion; Bank of America Corporation – $33.4 billion and The Coca-Cola Company – $22.1 billion).