Fair Value Measurements - Summary of assets and liabilities measured at fair value (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Assets:
Available-for-sale debt securities	$ 24,654	$ 29,587
Total	5,480	5,465
Fair Value, Measurements, Recurring \| Level 1
Assets:
Total	9,364	9,526
Liabilities:
Total	126	116
Fair Value, Measurements, Recurring \| Level 1 \| Corporate debt securities
Assets:
Available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring \| Level 1 \| U.S. treasury securities
Assets:
Available-for-sale debt securities	3,884	4,061
Fair Value, Measurements, Recurring \| Level 1 \| Residential mortgage and asset-backed securities
Assets:
Available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring \| Level 1 \| U.S. government agencies securities
Assets:
Available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Certificates of deposit
Assets:
Available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Non-U.S. government securities
Assets:
Available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Money market funds
Assets:
Marketable equity securities	4,674	4,714
Fair Value, Measurements, Recurring \| Level 1 \| Equity securities
Assets:
Marketable equity securities	680	635
Fair Value, Measurements, Recurring \| Level 1 \| Deferred compensation plan
Assets:
Marketable equity securities	126	116
Liabilities:
Deferred compensation plan	126	116
Fair Value, Measurements, Recurring \| Level 1 \| Foreign currency derivative contracts
Assets:
Foreign currency derivative contracts	0	0
Liabilities:
Foreign currency derivative contracts	0	0
Fair Value, Measurements, Recurring \| Level 2
Assets:
Total	20,774	25,539
Liabilities:
Total	110	93
Fair Value, Measurements, Recurring \| Level 2 \| Corporate debt securities
Assets:
Available-for-sale debt securities	11,972	14,747
Fair Value, Measurements, Recurring \| Level 2 \| U.S. treasury securities
Assets:
Available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring \| Level 2 \| Residential mortgage and asset-backed securities
Assets:
Available-for-sale debt securities	3,406	4,058
Fair Value, Measurements, Recurring \| Level 2 \| U.S. government agencies securities
Assets:
Available-for-sale debt securities	925	926
Fair Value, Measurements, Recurring \| Level 2 \| Certificates of deposit
Assets:
Available-for-sale debt securities	3,908	5,131
Fair Value, Measurements, Recurring \| Level 2 \| Non-U.S. government securities
Assets:
Available-for-sale debt securities	559	664
Fair Value, Measurements, Recurring \| Level 2 \| Money market funds
Assets:
Marketable equity securities	0	0
Fair Value, Measurements, Recurring \| Level 2 \| Equity securities
Assets:
Marketable equity securities	0	0
Fair Value, Measurements, Recurring \| Level 2 \| Deferred compensation plan
Assets:
Marketable equity securities	0	0
Liabilities:
Deferred compensation plan	0	0
Fair Value, Measurements, Recurring \| Level 2 \| Foreign currency derivative contracts
Assets:
Foreign currency derivative contracts	4	13
Liabilities:
Foreign currency derivative contracts	110	93
Fair Value, Measurements, Recurring \| Level 3
Assets:
Total	0	0
Liabilities:
Total	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Corporate debt securities
Assets:
Available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring \| Level 3 \| U.S. treasury securities
Assets:
Available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Residential mortgage and asset-backed securities
Assets:
Available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring \| Level 3 \| U.S. government agencies securities
Assets:
Available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Certificates of deposit
Assets:
Available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Non-U.S. government securities
Assets:
Available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Money market funds
Assets:
Marketable equity securities	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Equity securities
Assets:
Marketable equity securities	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Deferred compensation plan
Assets:
Marketable equity securities	0	0
Liabilities:
Deferred compensation plan	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Foreign currency derivative contracts
Assets:
Foreign currency derivative contracts	0	0
Liabilities:
Foreign currency derivative contracts	0	0
Fair Value, Measurements, Recurring \| Estimate of Fair Value Measurement
Assets:
Total	30,138	35,065
Liabilities:
Total	236	209
Fair Value, Measurements, Recurring \| Estimate of Fair Value Measurement \| Corporate debt securities
Assets:
Available-for-sale debt securities	11,972	14,747
Fair Value, Measurements, Recurring \| Estimate of Fair Value Measurement \| U.S. treasury securities
Assets:
Available-for-sale debt securities	3,884	4,061
Fair Value, Measurements, Recurring \| Estimate of Fair Value Measurement \| Residential mortgage and asset-backed securities
Assets:
Available-for-sale debt securities	3,406	4,058
Fair Value, Measurements, Recurring \| Estimate of Fair Value Measurement \| U.S. government agencies securities
Assets:
Available-for-sale debt securities	925	926
Fair Value, Measurements, Recurring \| Estimate of Fair Value Measurement \| Certificates of deposit
Assets:
Available-for-sale debt securities	3,908	5,131
Fair Value, Measurements, Recurring \| Estimate of Fair Value Measurement \| Non-U.S. government securities
Assets:
Available-for-sale debt securities	559	664
Fair Value, Measurements, Recurring \| Estimate of Fair Value Measurement \| Money market funds
Assets:
Marketable equity securities	4,674	4,714
Fair Value, Measurements, Recurring \| Estimate of Fair Value Measurement \| Equity securities
Assets:
Marketable equity securities	680	635
Fair Value, Measurements, Recurring \| Estimate of Fair Value Measurement \| Deferred compensation plan
Assets:
Marketable equity securities	126	116
Liabilities:
Deferred compensation plan	126	116
Fair Value, Measurements, Recurring \| Estimate of Fair Value Measurement \| Foreign currency derivative contracts
Assets:
Foreign currency derivative contracts	4	13
Liabilities:
Foreign currency derivative contracts	$ 110	$ 93

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- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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Amount of debt securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 2
-Subparagraph (b)-(g)
-URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803

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+ References

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+ References

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Fair value of financial and nonfinancial obligations.

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