Accumulated Other Comprehensive Loss - Schedule of Components of Other Comprehensive Loss (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended									3 Months Ended		6 Months Ended		3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2013	Mar. 31, 2013	Jun. 30, 2014 Treasury Lock [Member]	Mar. 31, 2014 Treasury Lock [Member]	Dec. 31, 2013 Treasury Lock [Member]	Jun. 30, 2013 Treasury Lock [Member]	Mar. 31, 2013 Treasury Lock [Member]	Dec. 31, 2012 Treasury Lock [Member]	Jun. 30, 2014 Pension and Other Postretirement Benefits [Member]	Jun. 30, 2013 Pension and Other Postretirement Benefits [Member]	Jun. 30, 2014 Pension and Other Postretirement Benefits [Member]	Jun. 30, 2013 Pension and Other Postretirement Benefits [Member]	Jun. 30, 2013 Interest Expense [Member] Treasury Lock [Member]	Jun. 30, 2013 Interest Expense [Member] Treasury Lock [Member]	Jun. 30, 2013 Commodity Derivatives [Member]	Jun. 30, 2013 Commodity Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ (33)	$ (34)	$ (48)	$ (42)	$ (9)	$ (9)	$ (9)	$ (9)	$ (10)	$ (10)	$ (24)	$ (31)	$ (25)	$ (32)			$ (1)	$ (6)
Amount of amortization of net prior service cost and actuarial loss reclassified to Other operation and maintenance expense											(3)	(1)	(2)	1
Amount of net pre-tax loss reclassified to loss from discontinued operations before income tax																	1	10
Amount of pre-tax loss reclassified to Interest expense															1	1
Income tax benefit																		(4)
Income tax expense (benefit)	1	1	(1)								1		1	(1)
Balance at end of period	$ (35)	$ (35)	$ (41)	$ (42)	$ (9)	$ (9)	$ (9)	$ (9)	$ (10)	$ (10)	$ (26)	$ (32)	$ (26)	$ (32)