Fair Value Measurements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gain (loss) on Level 3 energy derivative assets	$ 12
Gains (loss) on Level 3 energy derivative liabilities		33
Loss on Level 3 energy derivative assets for purchased power costs	46
Gain (loss) on Level 3 energy derivative liabilities for purchased power costs		29
Fair value, assets measured on recurring basis, change in unrealized gain (loss) included in other income	12	33
Fair value, assets measured on recurring basis, change in unrealized gain (loss) included in purchased power costs	$ 46	$ 15