Fair value measurements - Narrative (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Aug. 31, 2019	Dec. 31, 2017	Dec. 31, 2019
Occidental Petroleum Corporation [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition of preferred and common stock warrants	$ 10,000,000
Occidental Petroleum Corporation [Member] | Preferred Stock [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments in equity securities, description			The Occidental Preferred is redeemable at Occidental’s option beginning in 2029.
Occidental Petroleum Corporation [Member] | Common Stock Warrants [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments in equity securities, description			The Occidental common stock warrants expire on the one-year anniversary on which no Occidental Preferred remains outstanding.
RBI [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investment redeemed, face amount		$ 3,000,000,000