Financial Risk Management Activities (Schedule Of Reconciliation For Derivative Activity Included In Accumulated Other Comprehensive Loss) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Pre-Tax Balance at Beginning of Period	$ 12	$ 54
Gain (Loss) Recognized in AOCI		28
Less: Gain Reclassified into Income	(6)	(70)
Pre-Tax Balance at End of Period	6	12
After_Tax Balance at Beginning of Period	7	31
Gain Recognized in AOCI, After-Tax		17
Less: Gain Reclassified into Income, After-Tax	(4)	(41)
After_Tax Balance at End of Period	$ 3	$ 7