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				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 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	$ (75)		$ (99)		$ (79)		$ (74)
Included in regulatory assets and liabilities or balancing accounts	(1)	[1]	19	[1]	3	[1]	(6)	[1]
Liability balance	$ (76)		$ (80)		$ (76)		$ (80)

[1] The costs related to price risk management activities are recoverable through customer rates, therefore, balancing account revenue is recorded for amounts settled and purchased and there is no impact to net income. Unrealized gains and losses are deferred in regulatory liabilities and assets.