Financial Risk Management Activities (Schedule Of Reconciliation For Derivative Activity Included In Accumulated Other Comprehensive Loss) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gain (Loss) in AOCI	$ 25	$ 27	$ 78	$ 46
(Gain) Loss into Income	(3)	7	(19)	31
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			1		3
(Gain) Loss into Income	(2)		(2)		0
Pre-Tax Balance at End of Period	2		2		3
After-Tax Balance at Beginning of Period			2	0	0
Gain (Loss) in AOCI	0	1	0	2	2
(Gain) Loss into Income	(1)	$ 0	(1)	$ 0	0
After-Tax Balance at End of Period	$ 1		$ 1		$ 2