Investments in equity securities (Parenthetical) (Detail) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) Company		Dec. 31, 2020 USD ($) Company
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 310,739	[1]	$ 281,170	[2]
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	25,400		18,300
Apple Inc. [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	128,400		120,400
Bank of America Corporation [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	43,900		31,300
The Coca-Cola Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 21,000		$ 21,900
Equity Securities [Member] | American Express Company, Apple Inc., Bank of America Corporation and The Coca-Cola Company [Member] | Investment Concentration [Member]
Summary of Investment Holdings [Line Items]
Concentration percentage	70.00%		68.00%
Number of companies in concentration percentage | Company	4		4

[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).