Derivatives - Effect of Commodity Derivative Instruments in Statements of Income (Detail) - Commodity derivatives - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Sep. 30, 2017	Jun. 30, 2019	Jun. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Loss	$ 18	$ (57)	$ (82)	$ (85)
Sales and other operating revenues
Derivative Instruments, Gain (Loss) [Line Items]
Loss	3	(1)	(17)	(2)
Cost of revenues
Derivative Instruments, Gain (Loss) [Line Items]
Loss	$ 15	$ (56)	$ (65)	$ (83)