Fair Value Measurements (Schedule Of Quantitative Information About Level 3 Fair Value Measurements) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, Fair Value Disclosure	$ 7	$ 12
Liabilities, Fair Value Disclosure	(6)	(5)
Power [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, Fair Value Disclosure	7	12
Liabilities, Fair Value Disclosure	(6)	(5)
Electric Load Contracts [Member] | Power [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, Fair Value Disclosure	2	1
Liabilities, Fair Value Disclosure	$ (5)	$ (3)
Valuation Technique used	Discounted Cash flow	Discounted Cash flow
Fair Value Measurement With Significant Unobservable Inputs	Historic Load Variability	Historic Load Variability
Electric Load Contracts [Member] | Power [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Historic Load Variability	0.00%	0.00%
Electric Load Contracts [Member] | Power [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Historic Load Variability	15.00%	10.00%
Gas Physical Contract [Member] | Power [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Average Historical Basis	(40.00%)	(40.00%)
Gas Physical Contract [Member] | Power [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Average Historical Basis	0.00%	(10.00%)
Various [Member] | Power [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, Fair Value Disclosure	$ 5	$ 11
Liabilities, Fair Value Disclosure	$ (1)	$ (2)