Employee Benefit Plans - Pension and Other Retiree Benefit Related Amounts (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Pension Benefits [Member]
Pension And Other Employee Benefit Plans [Line Items]
Regulatory Pension Asset Included in Other Regulatory Assets	$ 129	$ 106
Accrued Benefit Liability Included in Accrued Employee Expenses	(1)	(1)
Accrued Benefit Liability Included in Pension and Other Retiree Benefits	(90)	(72)
Accumulated Other Comprehensive Loss (related to SERP)	4	2
Net Amount Recognized	42	35
Other Retiree Benefits [Member]
Pension And Other Employee Benefit Plans [Line Items]
Regulatory Pension Asset Included in Other Regulatory Assets	10	8
Accrued Benefit Liability Included in Accrued Employee Expenses	(2)	(2)
Accrued Benefit Liability Included in Pension and Other Retiree Benefits	(69)	(66)
Net Amount Recognized	$ (61)	$ (60)