Accumulated Other Comprehensive Loss - Schedule of Components of Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of the period	$ (42)	$ (35)	$ (46)	$ (34)
Amount of amortization of net prior service cost and actuarial loss reclassified to Other operation and maintenance expense	2	2	7
Income tax expense (benefit)	2		3	(1)
Balance at end of the period	(41)	(33)	(41)	(33)
Pension and Other Postretirement Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of the period	(33)	(26)	(37)	(25)
Amount of amortization of net prior service cost and actuarial loss reclassified to Other operation and maintenance expense	2	2	7
Income tax expense (benefit)	2		3	(1)
Balance at end of the period	(33)	(24)	(33)	(24)
Treasury Lock [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of the period	(9)	(9)	(9)	(9)
Balance at end of the period	(8)	$ (9)	(8)	$ (9)
Treasury Lock [Member] | Interest Expense [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount of pre-tax loss reclassified to Interest expense	$ 1		$ 1