Financial Risk Management Activities (Schedule Of Reconciliation For Derivative Activity Included In Accumulated Other Comprehensive Loss) (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gain (Loss) in AOCI	$ (16)	$ 30
(Gain) Loss into Income	10	(9)
Cash Flow Hedges [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Pre-Tax Balance at Beginning of Period	0	3	$ 3
Gain (Loss) in AOCI	0		0
(Gain) Loss into Income	0		(3)
Pre-Tax Balance at End of Period	0		0
After-Tax Balance at Beginning of Period	0	2	2
Gain (Loss) in AOCI	0	0	0
(Gain) Loss into Income	0	$ 0	(2)
After-Tax Balance at End of Period	$ 0		$ 0