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				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
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	$ (80)		$ (280)		$ (74)		$ (399)
Included in regulatory assets and liabilities or balancing accounts	(4)	[1]	(1)	[1]	(10)	[1]	118	[1]
Liability balance	$ (84)		$ (281)		$ (84)		$ (281)

[1] Price risk management activity is recoverable through customer rates. Therefore, net income was not impacted by realized amounts. Unrealized gains and losses are deferred in regulatory liabilities and assets.