Investments in equity securities (Parenthetical) (Detail) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2021 USD ($) Company		Dec. 31, 2020 USD ($) Company
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 307,942	[1]	$ 281,170	[2]
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	25,100		18,300
Apple Inc. [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	124,300		120,400
Bank of America Corporation [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	42,600		31,300
The Coca-Cola Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 21,600		$ 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 – $25.1 billion; Apple Inc. – $124.3 billion; Bank of America Corporation – $42.6 billion and The Coca-Cola Company – $21.6 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).