Derivatives And Risk Management (Summary Of Before Tax Effect Of Cash Flow Hedges On Consolidated Financial Statements) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	$ 81	$ (34)	$ (48)	$ (105)
Derivative Instrument, Gain (Loss) on Derivatives, Settled	(10)	49	20	135
Total gain (loss) on derivatives	71	15	(28)	30
Fixed Price Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	23	(37)	(17)	(110)
Derivative Instrument, Gain (Loss) on Derivatives, Settled	(9)	49	7	143
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	5
Three-way Costless-collars [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	1		(1)
Basis Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	31	1	27	(4)
Derivative Instrument, Gain (Loss) on Derivatives, Settled			4	(6)
Sold Call Options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled	21	3	(54)	11
Interest Rate Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instrument, Gain (Loss) on Derivatives, Unsettled		$ (1)	(3)	(2)
Derivative Instrument, Gain (Loss) on Derivatives, Settled	$ (1)		$ (2)	$ (2)