Investments (Securities In A Continuous Unrealized Loss Position) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Schedule of Short-term Investments [Table] [Line Items]
Fair Value available-for-sale securities Less than 12 Months	$ 70	$ 597
Unrealized losses, available-for-sale securities, Less than 12 Months	0	(1)
Fair value available-for-sale securities 12 Months or More	834	943
Unrealized losses, available-for-sale securities, 12 Months or More	(6)	(12)
Total Fair Value available-for-sale securities	904	1,540
Unrealized losses, available-for-sale securities, Total	(6)	(13)
U.S. Treasury And Agency Debt Securities [Member]
Schedule of Short-term Investments [Table] [Line Items]
Fair Value available-for-sale securities Less than 12 Months	13	84
Unrealized losses, available-for-sale securities, Less than 12 Months	0	0
Fair value available-for-sale securities 12 Months or More	412	446
Unrealized losses, available-for-sale securities, 12 Months or More	(4)	(6)
Total Fair Value available-for-sale securities	425	530
Unrealized losses, available-for-sale securities, Total	(4)	(6)
Corporate And Other Debt Securities [Member]
Schedule of Short-term Investments [Table] [Line Items]
Fair Value available-for-sale securities Less than 12 Months	15	108
Unrealized losses, available-for-sale securities, Less than 12 Months	0	(1)
Fair value available-for-sale securities 12 Months or More	233	254
Unrealized losses, available-for-sale securities, 12 Months or More	(1)	(3)
Total Fair Value available-for-sale securities	248	362
Unrealized losses, available-for-sale securities, Total	(1)	(4)
Certificates of Deposit and Commercial Paper [Member]
Schedule of Short-term Investments [Table] [Line Items]
Fair Value available-for-sale securities Less than 12 Months	8	295
Unrealized losses, available-for-sale securities, Less than 12 Months	0	0
Fair value available-for-sale securities 12 Months or More	0	0
Unrealized losses, available-for-sale securities, 12 Months or More	0	0
Total Fair Value available-for-sale securities	8	295
Unrealized losses, available-for-sale securities, Total	0	0
Asset-Backed Securities [Member]
Schedule of Short-term Investments [Table] [Line Items]
Fair Value available-for-sale securities Less than 12 Months	33	93
Unrealized losses, available-for-sale securities, Less than 12 Months	0	0
Fair value available-for-sale securities 12 Months or More	157	179
Unrealized losses, available-for-sale securities, 12 Months or More	(1)	(2)
Total Fair Value available-for-sale securities	190	272
Unrealized losses, available-for-sale securities, Total	(1)	(2)
Municipal Debt Securities [Member]
Schedule of Short-term Investments [Table] [Line Items]
Fair Value available-for-sale securities Less than 12 Months	1	17
Unrealized losses, available-for-sale securities, Less than 12 Months	0	0
Fair value available-for-sale securities 12 Months or More	32	64
Unrealized losses, available-for-sale securities, 12 Months or More	0	(1)
Total Fair Value available-for-sale securities	33	81
Unrealized losses, available-for-sale securities, Total	$ 0	$ (1)