Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Fair Value Measurements [Abstract]
Gain (loss) on Level 3 energy derivative assets	$ (3)
Gains (loss) on Level 3 energy derivative liabilities		(12)
Loss on Level 3 energy derivative assets for purchased power costs	(43)
Gain (loss) on Level 3 energy derivative liabilities for purchased power costs		27
Fair value, assets measured on recurring basis, change in unrealized gain (loss) included in other income	(3)	(12)
Fair value, assets measured on recurring basis, change in unrealized gain (loss) included in purchased power costs	$ (7)	$ 29