Fair Value Measurements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gain (loss) on Level 3 energy derivative assets	$ 2		$ 11
Gains (loss) on Level 3 energy derivative liabilities		10		35
Loss on Level 3 energy derivative assets for purchased power costs	2		42
Gain (loss) on Level 3 energy derivative liabilities for purchased power costs		12		6
Fair value, assets measured on recurring basis, change in unrealized gain (loss) included in other income	2	10	11	35
Fair value, assets measured on recurring basis, change in unrealized gain (loss) included in purchased power costs	16	5
Change in fair value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loss on Level 3 energy derivative assets for purchased power costs			40
Gain (loss) on Level 3 energy derivative liabilities for purchased power costs				$ 31