Derivative Financial Instruments (Gains And Losses Reported In Earnings Related To Commodity Derivative Instruments) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Derivative Financial Instruments Gains And Losses Reported In Earnings Related To Commodity Derivative Instruments [Line Items]
Net settlements received from (paid on) derivatives	$ 28	$ 259
Gain (loss) on derivatives	286	81
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	31	252
Gain (loss) on derivatives	266	72
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	(3)	7
Gain (loss) on derivatives	$ 20	$ 9