Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Fair Value Measurements [Abstract]
Gain (loss) on Level 3 energy derivative assets	$ (33)
Gains (loss) on Level 3 energy derivative liabilities		(37)
Loss on Level 3 energy derivative assets for purchased power costs	(29)
Gain (loss) on Level 3 energy derivative liabilities for purchased power costs		(43)
Fair value, assets measured on recurring basis, change in unrealized gain (loss) included in other income	(33)	(37)
Fair value, assets measured on recurring basis, change in unrealized gain (loss) included in purchased power costs	$ (15)	$ (24)