Fair value measurements - Narrative (Detail) - Occidental Corporation [Member] - USD ($) $ in Billions	1 Months Ended	3 Months Ended
	Aug. 31, 2019	Mar. 31, 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Acquisition of preferred and common stock warrants	$ 10
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
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