Fair Value Measurements (Narrative) (Detail) (USD $)	3 Months Ended	6 Months Ended			6 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014 Power [Member]	Jun. 30, 2013 Derivative [Member] Power [Member]	Jun. 30, 2014 Derivative [Member] Power [Member]	Jun. 30, 2013 Derivative [Member] Power [Member]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)					$ 1,000,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(1,000,000)	58,000,000	10,000,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net						6,000,000	(3,000,000)	4,000,000
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	$ 13,000,000	$ 13,000,000		$ (35,000,000)