Financial Risk Management Activities (Schedule Providing Credit Risk From Others, Net Of Collateral) (Detail) - Power [Member] $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($) Counterparty
Derivative [Line Items]
Current Exposure	$ 176
Securities held as Collateral	13
Net exposure	163
Number of Counterparties greater than 10%	$ 2
Number Of Active Counterparties On Credit Risk Derivatives | Counterparty	145
Investment Grade - External Rating [Member]
Derivative [Line Items]
Percentage Of Credit Exposure	98.00%
Investment Grade [Member]
Derivative [Line Items]
Current Exposure	$ 172
Securities held as Collateral	12
Net exposure	160
Number of Counterparties greater than 10%	2
Amount Of Net Credit Exposure Greater Than Ten Percent	74	[1]
External Credit Rating, Non Investment Grade [Member]
Derivative [Line Items]
Current Exposure	4
Securities held as Collateral	1
Amount Of Net Credit Exposure Greater Than Ten Percent	0
Non-Investment Grade [Member]
Derivative [Line Items]
Net exposure	3
Number of Counterparties greater than 10%	0
Cash [Member]
Derivative [Line Items]
Securities held as Collateral	1
Letter of Credit [Member]
Derivative [Line Items]
Securities held as Collateral	$ 12

[1] Represents net exposure of $56 million