Derivative Financial Instruments (Gains And Losses Reported In Earnings Related To Commodity Derivative Instruments) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivative Financial Instruments Gains And Losses Reported In Earnings Related To Commodity Derivative Instruments [Line Items]
Net settlements received from (paid on) derivatives	$ 68	$ 168	$ 96	$ 427
Gain (loss) on derivatives	209	(298)	495	(217)
Oil Commodity Derivative [Member]
Derivative Financial Instruments Gains And Losses Reported In Earnings Related To Commodity Derivative Instruments [Line Items]
Net settlements received from (paid on) derivatives	70	160	101	412
Gain (loss) on derivatives	199	(281)	465	(209)
Natural Gas Commodity Derivative [Member]
Derivative Financial Instruments Gains And Losses Reported In Earnings Related To Commodity Derivative Instruments [Line Items]
Net settlements received from (paid on) derivatives	(2)	8	(5)	15
Gain (loss) on derivatives	$ 10	$ (17)	$ 30	$ (8)