Financial Risk Management Activities (Schedule Providing Credit Risk From Others, Net Of Collateral) (Detail) (Power [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Derivative [Line Items]
Current Exposure	$ 327
Securities held as Collateral	60
Net exposure	326
Number of Counterparties greater than 10%	1
Net Exposure of Counterparties greater than 10%	200
Investment Grade - External Rating [Member]
Derivative [Line Items]
Current Exposure	311
Securities held as Collateral	60
Net exposure	310
Number of Counterparties greater than 10%	1
Net Exposure of Counterparties greater than 10%	200
Non-Investment Grade - External Rating [Member]
Derivative [Line Items]
Current Exposure	12
Securities held as Collateral	0
Net exposure	12
Number of Counterparties greater than 10%	0
Net Exposure of Counterparties greater than 10%	0
Investment Grade - No External Rating [Member]
Derivative [Line Items]
Current Exposure	3
Securities held as Collateral	0
Net exposure	3
Number of Counterparties greater than 10%	0
Net Exposure of Counterparties greater than 10%	0
Non-Investment Grade - No External Rating [Member]
Derivative [Line Items]
Current Exposure	1
Securities held as Collateral	0
Net exposure	1
Number of Counterparties greater than 10%	0
Net Exposure of Counterparties greater than 10%	$ 0