Investments Available-for-sale securities in a continuous unrealized loss position (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	$ 1,165	$ 1,347
Unrealized losses, less than 12 months	7	7
Fair value, 12 months or more	362	3
Unrealized losses, 12 months or more	4	0
Total fair value	1,527	1,350
Total unrealized loss	11	7
U.S. Treasury And Agency Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	519	526
Unrealized losses, less than 12 months	4	3
Fair value, 12 months or more	124	0
Unrealized losses, 12 months or more	2	0
Total fair value	643	526
Total unrealized loss	6	3
Corporate And Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	297	342
Unrealized losses, less than 12 months	1	2
Fair value, 12 months or more	126	1
Unrealized losses, 12 months or more	1	0
Total fair value	423	343
Total unrealized loss	2	2
Commercial Paper and Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	49	185
Unrealized losses, less than 12 months	0	0
Fair value, 12 months or more	0	0
Unrealized losses, 12 months or more	0	0
Total fair value	49	185
Total unrealized loss	0	0
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	193	206
Unrealized losses, less than 12 months	1	1
Fair value, 12 months or more	94	1
Unrealized losses, 12 months or more	1	0
Total fair value	287	207
Total unrealized loss	2	1
Municipal Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	107	88
Unrealized losses, less than 12 months	1	1
Fair value, 12 months or more	18	1
Unrealized losses, 12 months or more	0	0
Total fair value	125	89
Total unrealized loss	$ 1	$ 1