Fair Value Measurements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gain (loss) on Level 3 energy derivative assets				$ 80
Other Income [Member] | Competitive energy businesses [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gain (loss) on Level 3 energy derivative assets	(2)		(2)
Gain (loss) on Level 3 energy derivative liabilities		(6)		(9)
Fair value, assets measured on recurring basis, change in unrealized gain (loss)	(2)	(6)	(2)	(9)
Purchased power expense [Member] | Competitive energy businesses [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gain (loss) on Level 3 energy derivative assets	(15)		4
Gain (loss) on Level 3 energy derivative liabilities		(1)		(44)
Fair value, assets measured on recurring basis, change in unrealized gain (loss)	$ (14)	$ 31	$ 2	$ 24