Financial Risk Management Activities (Schedule Of Derivative Instruments Not Designated As Hedging Instruments And Impact On Results Of Operations) (Detail) - Energy-Related Contracts [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Pre-Tax Gain (Loss) Recognized in Income on Derivatives	$ 78	$ 218
Operating Revenues [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Pre-Tax Gain (Loss) Recognized in Income on Derivatives	83	216
Energy Costs [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Pre-Tax Gain (Loss) Recognized in Income on Derivatives	$ (5)	$ 2