Fair Value Measurements (Narrative) (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	$ (12,000,000)	$ 59,000,000
Net assets measured at fair value on a recurring basis	3,100,000,000		2,500,000,000
Net assets measured at fair value on a recurring basis measured using unobservable input and as Level 3	9,000,000		1,000,000
Power [Member]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(9,000,000)	(5,000,000)
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Trading Revenue	$ 3,000,000	$ (64,000,000)