Fair Value Measurements (Narrative) (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	$ (4,000,000)	$ (1,000,000)	$ 54,000,000	$ 9,000,000
Net assets measured at fair value on a recurring basis	2,500,000,000		2,500,000,000		1,900,000,000
Net assets measured at fair value on a recurring basis measured using unobservable input and as Level 3	(8,000,000)		(8,000,000)		(46,000,000)
Power [Member]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(12,000,000)	1,000,000	(11,000,000)	(7,000,000)
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Trading Revenue	(8,000,000)	1,000,000	(66,000,000)	(16,000,000)
Derivative [Member] | Power [Member]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net			$ (3,000,000)	$ 4,000,000