Fair Value Measurements - Level 3 (Details) (Commodity Derivative [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Commodity Derivative [Member]
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 2	$ (351)	$ 1	$ (345)
Changes in fair value recognized in net regulatory assets	0	94	1	79
Settlements	0	17	0	26
Ending balance	$ 2	$ (240)	$ 2	$ (240)