Accumulated Other Comprehensive Loss - Schedule of Components of Other Comprehensive Loss (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning of period	$ (48)	$ (63)	$ (106)
Income Tax expense (benefit)	6	(6)	(4)
Balance, end of period	(34)	(48)	(63)
Pension and Other Postretirement Benefit [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning of period	(32)	(24)	(17)
Amount of amortization of net prior service cost and actuarial loss reclassified to Other operation and maintenance expense	5	5	3
Amount of net prior service cost and actuarial gain (loss) arising during the year	8	(19)	(14)
Income Tax expense (benefit)	6	(6)	(4)
Balance, end of period	(25)	(32)	(24)
Treasury Lock [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning of period	(10)	(10)	(11)
Income Tax expense (benefit)
Balance, end of period	(9)	(10)	(10)
Treasury Lock [Member] | Interest Expense [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount of net pre-tax loss reclassified to (from) expense	1		1
Commodity Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning of period	(6)	(29)	(78)
Amount of net pre-tax loss reclassified to (Loss) income from discontinued operations before income tax	10	39	81
Income tax benefit	4	16	32
Balance, end of period		$ (6)	$ (29)