Fair Value Measurements (Schedule Of Unrealized Gains (Losses) Related To Available-For-Sale Investments) (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Jun. 30, 2017		Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	[1]	$ 1,797		$ 1,749
Total Unrealized Gains	[1]	1,334		1,205
Total Unrealized Losses	[1]	(8)		(15)
Total Fair Value	[1]	3,123		2,939
Amount primarily related to deferred taxes on appreciation of investment value		390	[2]	333	[3]
Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		10		9
Total Unrealized Gains		0		0
Total Unrealized Losses		0		0
Total Fair Value		10		9
Other Fixed-Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,260		1,156
Total Unrealized Gains		57		48
Total Unrealized Losses		(6)		(12)
Total Fair Value		1,311		1,192
Global equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		527		584
Total Unrealized Gains		1,277		1,157
Total Unrealized Losses		(2)		(3)
Total Fair Value		$ 1,802		$ 1,738

[1] Represents amounts before deducting $390 million and $333 million at June 30, 2017 and December 31, 2016, respectively, primarily related to deferred taxes on appreciation of investment value.

[2] Represents amount before deducting $390 million, primarily related to deferred taxes on appreciation of investment value.

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