Fair Value Measurements (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 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	$ (4)	$ (1)	$ (16)	$ 58
Net assets measured at fair value on a recurring basis	2,700		2,700		$ 2,500
Net assets measured at fair value on a recurring basis measured using unobservable input and as Level 3	8		8		$ 13
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)	0	6	(9)	1
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Trading Revenue	$ 5	$ 6	$ 8	(58)
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)