Marketable Securities (Summary of Securities Held) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 985	$ 435
Gross Unrealized Holding Gains	2	1
Gross Unrealized Holding Losses	0	(2)
Fair Value	987	434
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	985	435
Gross Unrealized Holding Gains	1	0
Gross Unrealized Holding Losses	0	(2)
Fair Value	986	433
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	33	114
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0	0
Fair Value	33	114
Corporate Bond Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	471	124
Gross Unrealized Holding Gains	1	0
Gross Unrealized Holding Losses	0	(1)
Fair Value	472	123
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		91
Gross Unrealized Holding Gains		0
Gross Unrealized Holding Losses		0
Fair Value		91
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5	28
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0	0
Fair Value	5	28
Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	449	73
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0	(1)
Fair Value	449	72
Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	27	5
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0	0
Fair Value	27	5
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	0
Gross Unrealized Holding Gains	1	1
Gross Unrealized Holding Losses	0	0
Fair Value	$ 1	$ 1

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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