Derivative Financial Instruments (Gains And Losses Reported In Earnings Related To Commodity Derivative Instruments) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 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	$ 30	$ 155	$ 126	$ 582
Gain (loss) on derivatives	(206)	41	289	(176)
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	28	154	129	566
Gain (loss) on derivatives	(205)	36	260	(173)
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	1	(3)	16
Gain (loss) on derivatives	$ (1)	$ 5	$ 29	$ (3)