Accumulated Other Comprehensive Loss - Schedule of Components of Other Comprehensive Loss (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ (46)	$ (34)	$ (48)
Amount of amortization of net prior service cost and actuarial loss reclassified to Other operation and maintenance expense	15	(20)	13
Amount of net pre-tax loss reclassified to Income (loss) from discontinued operations before income tax	1	1	1
Balance at end of period	(36)	(46)	(34)
Pension and Other Postretirement Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(37)	(25)	(32)
Amount of amortization of net prior service cost and actuarial loss reclassified to Other operation and maintenance expense	6	5	5
Amount of net prior service cost and actuarial gain (loss) arising during the year	9	(25)	8
Income tax (benefit) expense	6	(8)	6
Balance at end of period	(28)	(37)	(25)
Treasury Lock [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(9)	(9)	(10)
Amount of pre-tax loss reclassified to Interest expense	1	1	1
Income tax (benefit) expense		1
Balance at end of period	$ (8)	$ (9)	(9)
Commodity Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period			(6)
Amount of net pre-tax loss reclassified to Income (loss) from discontinued operations before income tax			10
Income tax (benefit) expense			$ 4