Fair Value (Details 1) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	$ 0	$ 1	$ 1	$ 2
Fair Value Measurements with Unobservable Inputs Reconciliation Gains and Losses Recorded in Regulatory Assets Liabilities	4	4	5	3
Purchases issuances and settlements [Abstract]
Settlements	(2)	(2)	(4)	(2)
Ending Balance	2	3	2	3
The amount of total gains (losses) included in net income attributed to the change in unrealized gains or losses related to assets still held at the end of the period	$ 2	$ 3	$ 2	$ 3