Regulatory Assets, Liabilities, And Balancing Accounts (Current Regulatory Balancing Accounts, Net) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	$ 764	$ 708
Distribution Revenue Adjustment Mechanism [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	428	223
Utility Generation [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	361	241
Public Purpose Programs [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	56	57
Hazardous Substance [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	45	97
Gas Fixed Cost [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	(30)	16
Energy Procurement [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	(161)	(48)
Other Current Balancing Accounts [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	148	227
Energy Recovery Bonds [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	$ (83)	$ (105)