Investments in equity securities (Parenthetical) (Detail) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2024 USD ($) Company		Dec. 31, 2023 USD ($) Company
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	$ 271,650	[1]	$ 353,842	[2]
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	41,100		28,400
Apple Inc. [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	69,900		174,300
Bank of America Corporation [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	31,700		34,800
Coca Cola Company [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	28,700		23,600
Chevron Corporation [Member]
Summary of Investment Holdings [Line Items]
Equity Securities, Fv-Ni	$ 17,500		$ 18,800
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	70.00%		79.00%
Number of companies in concentration percentage | Company	5		5

[1] Approximately 70% of the aggregate fair value was concentrated in five companies (American Express Company – $41.1 billion; Apple Inc. – $69.9 billion; Bank of America Corporation – $31.7 billion; The Coca-Cola Company – $28.7 billion and Chevron Corporation – $17.5 billion).

[2] 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).