Accumulated Other Comprehensive Loss - Schedule of Components of Other Comprehensive Loss (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning of period	$ (41)	$ (36)	$ (42)	$ (34)
Income Tax expense (benefit)		2	1	(1)
Other comprehensive income (loss) from continuing operations, net of income taxes	1		2	(2)
Balance, end of period	(40)	(36)	(40)	(36)
Treasury Lock [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning of period	(9)	(10)	(10)	(10)
Total net pre-tax loss reclassified to (from) income		1	1	1
Income Tax expense (benefit)		1		1
Other comprehensive income (loss) from continuing operations, net of income taxes			1
Balance, end of period	(9)	(10)	(9)	(10)
Treasury Lock [Member] | Interest Expense [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total net pre-tax loss reclassified to (from) income		1	1	1
Pension and Other Postretirement Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning of period	(32)	(26)	(32)	(24)
Total net pre-tax loss reclassified to (from) income	1	1	2	(4)
Income Tax expense (benefit)		1	1	(2)
Other comprehensive income (loss) from continuing operations, net of income taxes	1		1	(2)
Balance, end of period	(31)	(26)	(31)	(26)
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 to (from) income	$ 1	$ 1	$ 2	$ (4)