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
	Mar. 31, 2017	Mar. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	$ 146	$ (21)
Derivative Instrument, Gain (Loss) on Derivatives, Settled	(30)	7
Total gain (loss) on derivatives	116	(14)
Fixed Price Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	118	20
Derivative Instrument, Gain (Loss) on Derivatives, Settled	(16)	4
Purchased Put Options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled		15
Two-way Costless-collars [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	31
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	57
Derivative Instrument, Gain (Loss) on Derivatives, Settled	(4)
Basis Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	(103)	(3)
Derivative Instrument, Gain (Loss) on Derivatives, Settled	(1)	4
Sold Call Options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	42	(50)
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)