Accumulated Other Comprehensive Loss - Schedule of Components of Other Comprehensive Loss (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended			3 Months Ended	6 Months Ended				3 Months Ended	6 Months Ended	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Treasury Lock [Member]	Jun. 30, 2013 Treasury Lock [Member]	Jun. 30, 2012 Treasury Lock [Member]	Mar. 31, 2012 Treasury Lock [Member]	Dec. 31, 2011 Treasury Lock [Member]	Jun. 30, 2013 Treasury Lock [Member] Interest Expense [Member]	Jun. 30, 2013 Treasury Lock [Member] Interest Expense [Member]	Jun. 30, 2013 Pension And Other Postretirement Benefit Plans [Member]	Jun. 30, 2012 Pension And Other Postretirement Benefit Plans [Member]	Jun. 30, 2013 Pension And Other Postretirement Benefit Plans [Member]	Jun. 30, 2012 Pension And Other Postretirement Benefit Plans [Member]	Jun. 30, 2013 Pension And Other Postretirement Benefit Plans [Member] Other Operation And Maintenance Expense [Member]	Jun. 30, 2012 Pension And Other Postretirement Benefit Plans [Member] Other Operation And Maintenance Expense [Member]	Jun. 30, 2013 Pension And Other Postretirement Benefit Plans [Member] Other Operation And Maintenance Expense [Member]	Jun. 30, 2012 Pension And Other Postretirement Benefit Plans [Member] Other Operation And Maintenance Expense [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning of period		$ (48)			$ (10)	$ (10)	$ (10)	$ (10)	$ (10)			$ (31)	$ (24)	$ (32)	$ (24)
Balance, beginning of period	(34)	(41)	(34)	(42)
Total net pre-tax loss reclassified (from) to income					(1)	(1)				(1)	(1)	(1)	(6)	1	(5)	(1)	(6)	1	(5)
Income Tax expense	4	(1)	3		0	0						0	(4)	(1)	(3)
Other comprehensive income from continuing operations, net of income taxes	(2)	1	(2)		1	1						(1)	(2)	0	(2)
Balance, end of period	$ (36)	$ (41)	$ (36)		$ (9)	$ (9)	$ (10)	$ (10)	$ (10)			$ (32)	$ (26)	$ (32)	$ (26)