Investments in equity securities - Narrative (Detail) - USD ($) $ / shares in Units, $ in Billions	9 Months Ended
	Sep. 30, 2023	Aug. 04, 2022	Dec. 31, 2019
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Investment owned, common shares outstanding	151,600,000
Investment owned, percent of common shares outstanding	20.80%
Occidental Petroleum Corporation [Member] | Investment Commitment [Member]
Summary of Investment Holdings [Line Items]
Number of common shares that can be purchased	83,860,000
Exercise price for warrants, per share	$ 59.62
Warrants exercisable period after the date preferred stock is fully redeemed	1 year
Occidental Petroleum Corporation [Member] | Occidental Common Stock [Member] | Minimum [Member]
Summary of Investment Holdings [Line Items]
Equity method investment ownership percentage		20.00%
Occidental Petroleum Corporation [Member] | Cumulative Perpetual Preferred Stock [Member] | Investment Commitment [Member]
Summary of Investment Holdings [Line Items]
Investment in preferred stock, liquidation value	$ 8.5
Investment in preferred stock, stated dividend rate	8.00%
Investment in preferred stock, redemption year start	2029
Investment in preferred stock, redemption price percentage of liquidation value	105.00%
Liquidation value of mandatory redemption notifications	$ 1.5
Investment in preferred stock, mandatory redemption price percentage of liquidation value	110.00%
Occidental Petroleum Corporation [Member] | Cumulative Perpetual Preferred Stock and Warrants [Member] | Investment Commitment [Member]
Summary of Investment Holdings [Line Items]
Investment commitment amount			$ 10.0