Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Purchased power expense
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, assets measured on recurring basis, change in unrealized gain (loss)	$ (4)	$ (3)	$ (2)	$ (8)
Competitive energy businesses
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gain (loss) on Level 3 energy derivative liabilities	$ (5)	$ (3)	$ (6)	$ (12)