Fair Value Measurements (Narrative) (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Payments to Acquire Available-for-sale Securities	$ 271,000,000	$ 511,000,000
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) Included in Trading Revenue	(24,000,000)	5,000,000
Current liabilities, fair value	27,000,000		46,000,000
Net assets measured at fair value on a recurring basis	1,800,000,000	1,800,000,000
Net assets measured at fair value on a recurring basis measured using unobservable input and as Level 3	57,000,000	11,000,000
Power [Member]
Payments to Acquire Available-for-sale Securities	256,000,000	385,000,000
Current liabilities, fair value	27,000,000		46,000,000
Energy Holdings [Member]
Non-Recourse Debt	18,000,000
Rabbi Trust [Member]
Payments to Acquire Available-for-sale Securities	13,000,000
Rabbi Trust [Member] | Power [Member]
Payments to Acquire Available-for-sale Securities	3,000,000
Rabbi Trust [Member] | Other Entity [Member]
Payments to Acquire Available-for-sale Securities	10,000,000
Minimum [Member] | MMBTU [Member] | Long-Term Gas Supply [Member]
Longer-Term Basis Range	0
Minimum [Member] | MW-Day [Member] | Long Term Electric Capacity [Member]
Forecasted capacity prices	100
Maximum [Member] | MMBTU [Member] | Long-Term Gas Supply [Member]
Longer-Term Basis Range	2
Maximum [Member] | MW-Day [Member] | Long Term Electric Capacity [Member]
Forecasted capacity prices	400
Derivative [Member] | Power [Member]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net	$ 2,000,000