Regulatory Assets, Liabilities, And Balancing Accounts (Narrative) (Details) (USD $)	Mar. 31, 2012	Dec. 31, 2011
Regulatory Assets [Line Items]
Current regulatory assets	$ 1,024,000,000	$ 1,090,000,000
Undepreciated conventional electromechanical meters regulatory assets recovery maximum (months)	12
Book value of undepreciated conventional electromechanical meters	50,000,000
Deferred income taxes regulatory asset recovery minimum (years)	1
Amortization of underlying retained generation regulatory assets	62,000,000
Amortization of underlying generation facilities expected to recovered (months)	12
Deferred income taxes regulatory asset recovery maximum (years)	44
Utility retained generation asset costs	1,200,000,000
Unrealized losses related to price risk management derivative instruments	12
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
Term of a terminated power purchase agreement (years)	30
Current regulatory liabilities	83,000,000	161,000,000
Period of time expected to receive authorized rate adjustments (months)	12
Period of time exceeded for regulatory balancing accounts to be recorded in other noncurrent assets (months)	12
Recovery of remediation costs, percentage	90.00%
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	227,000,000	336,000,000
Price Risk Management Derivative Instruments [Member]
Regulatory Assets [Line Items]
Current regulatory assets	$ 488,000,000