Regulatory Assets, Liabilities, And Balancing Accounts (Narrative) (Detail) (USD $)	Sep. 30, 2012 Year	Dec. 31, 2011
Regulatory Assets [Line Items]
Current regulatory assets	$ 567,000,000	$ 1,090,000,000
Deferred income taxes regulatory asset recovery minimum (years)	1
Deferred income taxes regulatory asset recovery maximum (years)	45
Utility retained generation asset costs	1,200,000,000
Weighted average remaining life of Utility's retained generation assets (years)	13
Environmental compliance costs regulatory asset recovery (years)	32
Price risk management regulatory asset recovery (years)	10
Recovery of costs related to debt reacquired or redeemed prior to maturity (years)	14
Current regulatory liabilities	379,000,000	161,000,000
Recovery of energy procurement costs, percentage	100.00%
Electromechanical Meters [Member]
Regulatory Assets [Line Items]
Expected recovery of regulated asset (years)	4
Energy Recovery Bonds [Member]
Regulatory Assets [Line Items]
Current regulatory assets	$ 0	$ 336,000,000