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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ (35)	$ (41)	$ (34)	$ (48)
Income tax benefit (expense)			1	(1)
Balance at end of period	(33)	(40)	(33)	(40)
Treasury Lock [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(9)	(9)	(9)	(10)
Income tax benefit (expense)
Balance at end of period	(9)	(9)	(9)	(9)
Treasury Lock [Member] | Interest Expense [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount of pre-tax loss reclassified to Interest expense				1
Pension and Other Postretirement Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(26)	(32)	(25)	(32)
Amount of amortization of net prior service cost and actuarial loss reclassified to Other operation and maintenance expense	2	1		2
Income tax benefit (expense)			1	(1)
Balance at end of period	(24)	(31)	(24)	(31)
Commodity Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period				(6)
Amount of net pre-tax loss reclassified to loss from discontinued operations before income tax				10
Income tax benefit (expense)				(4)
Balance at end of period