Fair Value Measurements (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	$ (2)	$ (4)	$ (18)	$ 54
Net assets measured at fair value on a recurring basis	2,300		2,300		$ 2,500
Net assets measured at fair value on a recurring basis measured using unobservable input and as Level 3	2		2		$ 8
Power [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	3	(12)	(6)	(11)
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Trading Revenue	$ 4	$ (8)	$ 12	(66)
Derivative [Member] | Power [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net				$ (3)