Fair Value Measurements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Non-utility Revenues [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, assets measured on recurring basis, change in unrealized gain (loss)				$ (2)
Purchased Power Costs [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, assets measured on recurring basis, change in unrealized gain (loss)	(2)	(5)	8	(3)
Competitive Energy Businesses [Member] | Non-utility Revenues [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gain (loss) on Level 3 energy derivative liabilities				(2)
Competitive Energy Businesses [Member] | Purchased Power Costs [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gain (loss) on Level 3 energy derivative liabilities	$ (5)	$ (4)	$ 35