Accumulated Other Comprehensive Loss - Schedule of Components of Other Comprehensive Loss (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ (34)	$ (48)	$ (63)
Income tax (benefit) expense	(7)	6	(6)
Balance at end of period	(46)	(34)	(48)
Pension and Other Postretirement Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(25)	(32)	(24)
Amount of amortization of net prior service cost and actuarial loss reclassified to Other operation and maintenance expense	5	5	5
Amount of net prior service cost and actuarial (loss) gain arising during the year	(25)	8	(19)
Income tax (benefit) expense	(8)	6	(6)
Balance at end of period	(37)	(25)	(32)
Treasury Lock [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(9)	(10)	(10)
Income tax (benefit) expense	1
Balance at end of period	(9)	(9)	(10)
Treasury Lock [Member] | Interest Expense [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount of pre-tax loss reclassified to Interest expense	1	1
Commodity Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		(6)	(29)
Amount of net pre-tax loss reclassified to (Loss) income from discontinued operations before income tax		10	39
Income tax (benefit) expense		4	16
Balance at end of period			$ (6)