Fair Value Measurements (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Gain (loss) on Level 3 energy derivative assets	$ (6)		$ (9)
Gains (loss) on Level 3 energy derivative liabilities		(13)		(25)
Loss on Level 3 energy derivative assets for purchased power costs	1		44
Gain (loss) on Level 3 energy derivative liabilities for purchased power costs		(9)		(18)
Fair value, assets measured on recurring basis, change in unrealized gain (loss) included in other income	(6)	(13)
Fair value, assets measured on recurring basis, change in unrealized gain (loss) included in purchased power costs	31	6
Change in Fair Value [Member]
Loss on Level 3 energy derivative assets for purchased power costs			24
Gain (loss) on Level 3 energy derivative liabilities for purchased power costs				$ 36