Fair Value Measurements (Details) - Financial Instruments - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financial Instruments
Short-term securities	$ 4,437	$ 4,895
Commercial paper	100	100
Carrying Value [Member]
Financial Instruments
Short-term securities	4,437	4,895
Debt	6,464	6,471
Commercial paper	100	100
Fair Value [Member]
Financial Instruments
Short-term securities	4,437	4,895
Debt	7,115	7,702
Commercial paper	100	100
Fair Value [Member] \| Level 1 [Member]
Financial Instruments
Short-term securities	710	1,238
Fair Value [Member] \| Level 2 [Member]
Financial Instruments
Short-term securities	3,692	3,622
Debt	7,115	7,702
Commercial paper	100	100
Fair Value [Member] \| Level 3 [Member]
Financial Instruments
Short-term securities	$ 35	$ 35

Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13)(2))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a)(3))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_CommercialPaper
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant