Investments in equity securities (Parenthetical) (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) Company		Dec. 31, 2019 USD ($) Company
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 281,170	[1]	$ 248,027	[2]
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	18,300		18,900
Apple Inc. [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	120,400		73,700
Bank of America Corporation [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	31,300		33,400
The Coca-Cola Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 21,900		$ 22,100
Equity Securities [Member] | Investment Concentration [Member]
Summary of Investment Holdings [Line Items]
Concentration percentage	68.00%		60.00%
Number of companies in concentration percentage | Company	4		4

[1] 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).

[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).