Fair Value Measurements (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	$ (15)	$ (12)
Net assets measured at fair value on a recurring basis	2,800		$ 3,100
Net assets measured at fair value on a recurring basis measured using unobservable input and as Level 3	21		$ 9
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)		(9)
Fair Value Assets And Liabilities Measured On Recurring Basis Gain Loss Included In Trading Revenue	$ 15	$ 3