Regulatory Assets, Liabilities, And Balancing Accounts (Current Regulatory Balancing Accounts, Net) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	$ 139		$ 302
Distribution Revenue Adjustment Mechanism [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	359		219
Utility Generation [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	321		117
Hazardous Substance [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	76		56
Public Purpose Program [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	(69)		(83)
Gas Fixed Cost [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	(81)		44
Energy Procurement [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	(31)		77
US Department Energy Settlement [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	(250)		(250)
Greenhouse Gas Allowance Auction Proceeds [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	(141)	[1]	0	[1]
Other Current Balancing Accounts [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	147		165
Energy Recovery Bonds [Member]
Regulatory Assets [Line Items]
Total regulatory balancing accounts, net	$ (192)		$ (43)

[1] The CARB has adopted regulations that established a state-wide, cap-and-trade program (effective January 1, 2013) that sets a gradually declining limit on the amount of GHGs that may be emitted each year. This balancing account is used to record proceeds collected by the Utility for GHG emission allowances associated with the cap-and-trade program. These amounts will be refunded to customers in future periods.