Fair Value Measurements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gain (loss) on Level 3 energy derivative assets				$ 95
Competitive energy businesses [Member] | Other Income [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gain (loss) on Level 3 energy derivative assets			(2)
Gain (loss) on Level 3 energy derivative liabilities		(2)		(11)
Fair value, assets measured on recurring basis, change in unrealized gain (loss)		(2)	(2)	(11)
Competitive energy businesses [Member] | Purchased power expense [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gain (loss) on Level 3 energy derivative assets	(4)
Gain (loss) on Level 3 energy derivative liabilities		2		(42)
Fair value, assets measured on recurring basis, change in unrealized gain (loss)	$ (5)	$ 16	$ (3)	$ 40