Accumulated Other Comprehensive Loss - Schedule of Components of Other Comprehensive Loss (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ (46)	$ (34)
Amount of amortization of net prior service cost and actuarial loss reclassified to Other operation and maintenance expense	2	1
Income tax expense (benefit)	1
Balance at end of period	(45)	(33)
Pension and Other Postretirement Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(37)	(25)
Amount of amortization of net prior service cost and actuarial loss reclassified to Other operation and maintenance expense	2	1
Income tax expense (benefit)	1
Balance at end of period	(36)	(24)
Treasury Lock [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(9)	(9)
Income tax expense (benefit)	0	0
Balance at end of period	(9)	(9)
Treasury Lock [Member] | Interest Expense [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount of pre-tax loss reclassified to Interest expense	$ 0	$ 0