Fair Value Measurements (Level 3 Reconciliation) (Details) (Fair Value Measurements, Level 3 [Member], Price Risk Management Instruments [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011
Fair Value Measurements, Level 3 [Member] | Price Risk Management Instruments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Liability balance as of January 1	$ (74)		$ (399)
Realized and unrealized gains (losses): Included in regulatory assets and liabilities or balancing accounts	(25)	[1]	87	[1]
Liability balance as of March 31	$ (99)		$ (312)

[1] Includes the effect of the contractual ability to settle contracts under master netting agreements and margin cash collateral.