Investments in equity securities (Parenthetical) (Detail) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2022 USD ($) Company		Dec. 31, 2021 USD ($) Company
Summary of Investment Holdings [Line Items]
Equity Securities Fv Ni	$ 306,167	[1]	$ 350,719	[2]
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Equity Securities Fv Ni	20,500		24,800
Apple Inc. [Member]
Summary of Investment Holdings [Line Items]
Equity Securities Fv Ni	126,500		161,200
Bank of America Corporation [Member]
Summary of Investment Holdings [Line Items]
Equity Securities Fv Ni	31,200		46,000
Coca Cola Company [Member]
Summary of Investment Holdings [Line Items]
Equity Securities Fv Ni	22,400		$ 23,700
Chevron Corporation [Member]
Summary of Investment Holdings [Line Items]
Equity Securities Fv Ni	$ 24,400
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	73.00%
Number of companies in concentration percentage | Company	5
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 73% of the aggregate fair value was concentrated in five companies (American Express Company – $20.5 billion; Apple Inc. – $126.5 billion; Bank of America Corporation – $31.2 billion; The Coca-Cola Company – $22.4 billion and Chevron Corporation – $24.4 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).