Financial Risk Management Activities (Schedule Of Reconciliation For Derivative Activity Included In Accumulated Other Comprehensive Loss) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gain (Loss) in AOCI	$ 23	$ 7	$ 53	$ 19
(Gain) Loss into Income	(7)	10	(16)	24
Cash Flow Hedges [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Pre-Tax Balance at Beginning of Period			3	0	$ 0
Gain (Loss) in AOCI			0		3
(Gain) Loss into Income			0		0
Pre-Tax Balance at End of Period	3		3		3
After-Tax Balance at Beginning of Period			2	0	0
Gain (Loss) in AOCI	0	(1)	0	1	2
(Gain) Loss into Income	0	$ 0	0	$ 0	0
After-Tax Balance at End of Period	$ 2		$ 2		$ 2