Investments in equity securities (Parenthetical) (Detail) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2023 USD ($) Company		Dec. 31, 2022 USD ($) Company
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	$ 353,409	[1]	$ 308,793	[2]
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	26,400		22,400
Apple Inc. [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	177,600		119,000
Bank of America Corporation [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	29,600		34,200
Coca Cola Company [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	24,100		25,400
Chevron Corporation [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	$ 19,400		$ 30,000
Equity Securities [Member] | American Express Company, Apple Inc., Bank of America Corporation, The Coca-Cola Company and Chevron Corporation [Member] | Investment Concentration [Member]
Summary of Investment Holdings [Line Items]
Concentration percentage	78.00%		75.00%
Number of companies in concentration percentage | Company	5		5

[1] Approximately 78% of the aggregate fair value was concentrated in five companies (American Express Company – $26.4 billion; Apple Inc. – $177.6 billion; Bank of America Corporation – $29.6 billion; The Coca-Cola Company – $24.1 billion and Chevron Corporation – $19.4 billion).

[2] Approximately 75% of the aggregate fair value was concentrated in five companies (American Express Company – $22.4 billion; Apple Inc. – $119.0 billion; Bank of America Corporation – $34.2 billion; The Coca-Cola Company – $25.4 billion and Chevron Corporation – $30.0 billion).