Investments in equity securities - Narrative (Detail) $ / shares in Units, $ in Billions	12 Months Ended
	Dec. 31, 2024 USD ($) Company $ / shares shares	Dec. 31, 2023 USD ($) Company
Summary of Investment Holdings [Line Items]
Investment, Type [Extensible Enumeration]	us-gaap:USTreasuryBillSecuritiesMember	us-gaap:USTreasuryBillSecuritiesMember
American Express Company [Member]
Summary of Investment Holdings [Line Items]
Investment owned, common shares outstanding | shares	151,600,000
Investment owned, percent of common shares outstanding	21.60%
American Express Company, Apple Inc., Bank of America Corporation, The Coca-Cola Company and Chevron Corporation [Member] | Equity Securities [Member] | Investment Concentration [Member]
Summary of Investment Holdings [Line Items]
Number of holdings in concentration percentage | Company	5	5
Concentration percentage	71.00%	79.00%
Occidental Petroleum Corporation [Member] | Investment Commitment [Member]
Summary of Investment Holdings [Line Items]
Investment in preferred stock, stated dividend rate	8.00%
Investment in preferred stock, liquidation value | $	$ 8.5
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, Type [Extensible Enumeration]	Cumulative Perpetual Preferred Stock [Member]	Cumulative Perpetual Preferred Stock [Member]
Number of common shares that can be purchased | shares	83,860,000
Exercise price for warrants, per share | $ / shares	$ 59.62
Warrants exercisable period after the date preferred stock is fully redeemed	1 year