Investments in equity securities (Parenthetical) (Detail) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021 USD ($) Company		Dec. 31, 2020 USD ($) Company
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 282,097	[1]	$ 281,170	[2]
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	21,400		18,300
Apple Inc. [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	110,900		120,400
Bank of America Corporation [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	40,000		31,300
The Coca-Cola Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 21,100		$ 21,900
Equity Securities [Member] | Investment Concentration [Member]
Summary of Investment Holdings [Line Items]
Concentration percentage	69.00%		68.00%
Number of companies in concentration percentage | Company	4		4

[1] Approximately 69% of the aggregate fair value was concentrated in four companies (American Express Company – $21.4 billion; Apple Inc. – $110.9 billion; Bank of America Corporation – $40.0 billion and The Coca-Cola Company – $21.1 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).