Derivatives And Risk Management (Summary Of Before Tax Effect Of Fair Value Hedges Not Designated For Hedge Accounting) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	$ 173	$ (108)	$ 319	$ (129)
Derivative Instrument, Gain (Loss) on Derivatives, Settled	(39)	23	(69)	30
Total gain (loss) on derivatives	134	(85)	250	(99)
Fixed Price Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	58	(60)	176	(40)
Derivative Instrument, Gain (Loss) on Derivatives, Settled	(9)	12	(25)	16
Purchased Put Options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled		(15)
Derivative Instrument, Gain (Loss) on Derivatives, Settled		11		11
Two-way Costless-collars [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	14	(5)	45	(5)
Derivative Instrument, Gain (Loss) on Derivatives, Settled			(3)
Three-way Costless-collars [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	31	(2)	88	(2)
Derivative Instrument, Gain (Loss) on Derivatives, Settled	(1)		(5)
Basis Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	60	(1)	(43)	(4)
Derivative Instrument, Gain (Loss) on Derivatives, Settled	(29)		(30)	4
Call Options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	11	$ (25)	53	(75)
Derivative Instrument, Gain (Loss) on Derivatives, Settled			$ (6)
Interest Rate Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	$ (1)			(3)
Derivative Instrument, Gain (Loss) on Derivatives, Settled				$ (1)