Accumulated Other Comprehensive Loss (Schedule Of Components Of Other Comprehensive Loss) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning of period	$ (45)	$ (50)
Income Tax expense	(2)	(3)
Balance, end of period	(42)	(45)
Commodity Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning of period	(3)	(16)
Total net pre-tax loss reclassified into income	3	7
Income Tax expense	(1)	(2)
Net change during period	2	5
Balance, end of period	(1)	(11)
Commodity Derivatives [Member] | Fuel And Purchased Energy Expense [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total net pre-tax loss reclassified into income	3	7
Commodity Derivatives [Member] | Revenue [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total net pre-tax loss reclassified into income
Treasury Lock [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning of period	(10)	(10)
Total net pre-tax loss reclassified into income
Income Tax expense
Net change during period
Balance, end of period	(10)	(10)
Treasury Lock [Member] | Interest Expense [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total net pre-tax loss reclassified into income
Pension And Other Postretirement Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning of period	(32)	(24)
Total net pre-tax loss reclassified into income	2	1
Income Tax expense	(1)	(1)
Net change during period	1
Balance, end of period	(31)	(24)
Pension And Other Postretirement Benefit Plans [Member] | Other Operation And Maintenance Expense [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total net pre-tax loss reclassified into income	$ 2	$ 1