Regulatory Assets, Liabilities, And Balancing Accounts (Current Regulatory Balancing Accounts, Net) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	$ 449	$ 708
Distribution Revenue Adjustment Mechanism [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	92	223
Utility Generation [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	68	241
Hazardous Substance [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	56	57
Public Purpose Program [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	53	97
Gas Fixed Cost [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	105	16
Energy Procurement [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	(19)	(48)
Other Current Balancing Accounts [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	151	227
Energy Recovery Bonds [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	$ (57)	$ (105)