Investments in equity securities - Narrative (Detail) - Investment Commitment [Member] $ / shares in Units, $ in Billions	3 Months Ended
	Mar. 31, 2021 USD ($) $ / shares shares
Summary of Investment Holdings [Line Items]
Investment commitment description	Our equity security investments include $10 billion in Occidental Corporation (“Occidental”) Cumulative Perpetual Preferred Stock with an aggregate liquidation value of $10 billion and warrants to purchase up to 83.86 million shares of Occidental common stock at an exercise price of $59.62 per share. The preferred stock accrues dividends at 8% per annum and is redeemable at the option of Occidental commencing in 2029 at a redemption price equal to 105% of the liquidation preference plus any accumulated and unpaid dividends or is mandatorily redeemable under specified events. Dividends on the preferred stock may be paid in cash or, at Occidental’s option, in shares of Occidental common stock. The warrants are exercisable in whole or in part until one year after the redemption of the preferred stock.
Occidental Corporation [Member]
Summary of Investment Holdings [Line Items]
Number of common shares that can be purchased | shares	83,860,000
Exercise price for warrants, per share | $ / shares	$ 59.62
Occidental Corporation [Member] | Cumulative Perpetual Preferred Stock and Warrants [Member]
Summary of Investment Holdings [Line Items]
Investment commitment amount	$ 10
Occidental Corporation [Member] | Cumulative Perpetual Preferred Stock [Member]
Summary of Investment Holdings [Line Items]
Investment in preferred stock, liquidation value	$ 10
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 preference	105.00%