Fair Value Measurements (Level 3 Reconciliation) (Detail) (Fair Value Measurements, Level 3 [Member], Price Risk Management Instruments [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Fair Value Measurements, Level 3 [Member] | Price Risk Management Instruments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Liability balance	$ (79)		$ (74)
Included in regulatory assets and liabilities or balancing accounts	4	[1]	(25)	[1]
Transfers out of level 3	0		0
Liability balance	$ (75)		$ (99)

[1] Price risk management activities are recoverable through customer rates, therefore, balancing account revenue is recorded for amounts settled and purchased and there is not impact to net income. Unrealized gains and losses are deferred in regulatory liabilities and assets.