Investments in equity securities (Parenthetical) (Detail) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2022 USD ($) Company		Dec. 31, 2021 USD ($) Company
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 390,538	[1]	$ 350,719	[2]
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	28,400		24,800
Apple Inc. [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	159,100		161,200
Bank of America Corporation [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	42,600		46,000
Chevron Corporation [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments	$ 25,900
Coca Cola Company [Member]
Summary of Investment Holdings [Line Items]
Fair value of investments			$ 23,700
Equity Securities [Member] | American Express Company, Apple Inc., Bank of America Corporation and Chevron Corporation [Member] | Investment Concentration [Member]
Summary of Investment Holdings [Line Items]
Concentration percentage	66.00%
Number of companies in concentration percentage | Company	4
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			73.00%
Number of companies in concentration percentage | Company			4

[1] Approximately 66% of the aggregate fair value was concentrated in four companies (American Express Company – $28.4 billion; Apple Inc. – $159.1 billion; Bank of America Corporation – $42.6 billion and Chevron Corporation – $25.9 billion).

[2] Approximately 73% of the aggregate fair value was concentrated in four companies (American Express Company – $24.8 billion; Apple Inc. – $161.2 billion; Bank of America Corporation – $46.0 billion and The Coca-Cola Company – $23.7 billion).