Investments in equity securities (Parenthetical) (Detail) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) Company		Dec. 31, 2019 USD ($) Company
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 245,317	[1]	$ 248,027	[2]
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	15,200		18,900
Apple Inc. [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	111,700		73,700
Bank of America Corporation [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	24,900		33,400
The Coca-Cola Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 19,700		$ 22,100
Equity Securities [Member] | Investment Concentration [Member]
Summary of Investment Holdings [Line Items]
Concentration percentage	70.00%		60.00%
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 – $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).