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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	$ 31	$ 81	$ 350	$ (48)
Derivative Instrument, Gain (Loss) on Derivatives, Settled	17	(10)	(52)	20
Total gain (loss) on derivatives	48	71	298	(28)
Fixed Price Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	(2)	23	174	(17)
Derivative Instrument, Gain (Loss) on Derivatives, Settled	7	(9)	(18)	7
Purchased Put Options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Settled				11
Two-way Costless-collars [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	3	5	48
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	(1)	1	87	(1)
Derivative Instrument, Gain (Loss) on Derivatives, Settled	1		(4)
Basis Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	24	31	(19)	27
Derivative Instrument, Gain (Loss) on Derivatives, Settled	9		(21)	4
Call Options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	6	21	59	(54)
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		1	(3)
Derivative Instrument, Gain (Loss) on Derivatives, Settled		$ (1)		$ (2)
Call Options - 2017 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Premium paid	$ 3		$ 3