Accumulated Other Comprehensive Loss - Schedule of Components of Other Comprehensive Loss (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			3 Months Ended
	Mar. 31, 2013	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Pension and Other Postretirement Benefit [Member]	Mar. 31, 2013 Pension and Other Postretirement Benefit [Member]	Mar. 31, 2014 Treasury Lock [Member]	Mar. 31, 2013 Treasury Lock [Member]	Mar. 31, 2014 Treasury Lock [Member] Interest Expense [Member]	Mar. 31, 2013 Treasury Lock [Member] Interest Expense [Member]	Mar. 31, 2014 Commodity Derivatives [Member]	Mar. 31, 2013 Commodity Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning of period	$ (48)	$ (33)	$ (34)	$ (25)	$ (32)	$ (9)	$ (10)				$ (6)
Amount of amortization of net prior service cost and actuarial loss reclassified to Other operation and maintenance expense				1	2
Amount of net pre-tax loss reclassified to (Loss) income from discontinued operations before income tax											8
Amount of pre-tax loss reclassified to Interest expense
Income tax benefit											(3)
Income tax benefit	1				(1)
Balance, end of period	$ (42)	$ (33)	$ (34)	$ (24)	$ (31)	$ (9)	$ (10)				$ (1)