Fair Value Measurements (Schedule Of Unrealized Gains Losses Related To Available-For-Sale Investments) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amount primarily related to deferred taxes on appreciation of investment value		$ 440	$ 333
Money Market Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		23	9
Total Unrealized Gains		0	0
Total Unrealized Losses		0	0
Total Fair Value		23	9
Global equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		524	584
Total Unrealized Gains		1,463	1,157
Total Unrealized Losses		(2)	(3)
Total Fair Value		1,985	1,738
Fixed-Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,252	1,156
Total Unrealized Gains		51	48
Total Unrealized Losses		(8)	(12)
Total Fair Value		1,295	1,192
Nuclear Decommissioning Trusts [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	[1]	1,799	1,749
Total Unrealized Gains	[1]	1,514	1,205
Total Unrealized Losses	[1]	(10)	(15)
Total Fair Value	[1]	$ 3,303	$ 2,939

[1] Represents amount before deducting $333 million, primarily related to deferred taxes on appreciation of investment value.