Investments in equity securities (Parenthetical) (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) Company		Dec. 31, 2022 USD ($) Company
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	$ 353,842	[1]	$ 308,793	[2]
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	28,400		22,400
Apple Inc. [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	174,300		119,000
Bank of America Corporation [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	34,800		34,200
Coca Cola Company [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	23,600		25,400
Chevron Corporation [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	$ 18,800		$ 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	79.00%		75.00%
Number of companies in concentration percentage | Company	5		5

[1] Approximately 79% of the aggregate fair value was concentrated in five companies (American Express Company – $28.4 billion; Apple Inc. – $174.3 billion; Bank of America Corporation – $34.8 billion; The Coca-Cola Company – $23.6 billion and Chevron Corporation – $18.8 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).