Accumulated Other Comprehensive Loss - Schedule of Components of Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of the period	$ (45)	$ (33)	$ (46)	$ (34)
Amount of amortization of net prior service cost and actuarial loss reclassified to Other operation and maintenance expense	3	(3)	5	(2)
Income tax expense (benefit)		(1)	1	(1)
Balance at end of the period	(42)	(35)	(42)	(35)
Pension and Other Postretirement Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of the period	(36)	(24)	(37)	(25)
Amount of amortization of net prior service cost and actuarial loss reclassified to Other operation and maintenance expense	3	(3)	5	(2)
Income tax expense (benefit)		(1)	1	(1)
Balance at end of the period	(33)	(26)	(33)	(26)
Treasury Lock [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of the period	(9)	(9)	(9)	(9)
Income tax expense (benefit)	0	0	0	0
Balance at end of the 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	$ 0	$ 0	$ 0	$ 0