Fair Value Measurements (Schedule Of Unrealized Gains (Losses) Related To Available-For-Sale Investments) (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2017		Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	[1]	$ 1,790		$ 1,749
Total Unrealized Gains	[1]	1,280		1,205
Total Unrealized Losses	[1]	(11)		(15)
Total Fair Value	[1]	3,059		2,939
Amount primarily related to deferred taxes on appreciation of investment value		358	[2]	333	[3]
Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		8		9
Total Unrealized Gains		0		0
Total Unrealized Losses		0		0
Total Fair Value		8		9
Other Fixed-Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,233		1,156
Total Unrealized Gains		50		48
Total Unrealized Losses		(9)		(12)
Total Fair Value		1,274		1,192
Global equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		549		584
Total Unrealized Gains		1,230		1,157
Total Unrealized Losses		(2)		(3)
Total Fair Value		$ 1,777		$ 1,738

[1] Represents amounts before deducting $358 million and $333 million at March 31, 2017 and December 31, 2016, respectively, primarily related to deferred taxes on appreciation of investment value.

[2] Represents amount before deducting $358 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.