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Fair Value Measurements - Summary of assets and liabilities measured at fair value (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets:
Available-for-sale debt securities	$ 23,453	$ 29,587
Total	6,102	5,465
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement
Assets:
Total	29,597	35,065
Liabilities:
Total	145	209
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | U.S. treasury securities
Assets:
Available-for-sale debt securities	3,075	4,061
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Certificates of deposit
Assets:
Available-for-sale debt securities	4,392	5,131
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | U.S. government agencies securities
Assets:
Available-for-sale debt securities	932	926
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Non-U.S. government securities
Assets:
Available-for-sale debt securities	260	664
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Corporate debt securities
Assets:
Available-for-sale debt securities	12,757	14,747
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Residential mortgage and asset-backed securities
Assets:
Available-for-sale debt securities	2,037	4,058
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Money market funds
Assets:
Marketable equity securities	5,138	4,714
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Equity securities
Assets:
Marketable equity securities	825	635
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Deferred compensation plan
Assets:
Marketable equity securities	139	116
Liabilities:
Deferred compensation plan	139	116
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Foreign currency derivative contracts
Assets:
Foreign currency derivative contracts	42	13
Liabilities:
Foreign currency derivative contracts	6	93
Fair Value, Measurements, Recurring | Level 1
Assets:
Total	9,177	9,526
Liabilities:
Total	139	116
Fair Value, Measurements, Recurring | Level 1 | U.S. treasury securities
Assets:
Available-for-sale debt securities	3,075	4,061
Fair Value, Measurements, Recurring | Level 1 | Certificates of deposit
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 | Non-U.S. government securities
Assets:
Available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring | Level 1 | Corporate debt securities
Assets:
Available-for-sale debt securities	0	0
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 | Money market funds
Assets:
Marketable equity securities	5,138	4,714
Fair Value, Measurements, Recurring | Level 1 | Equity securities
Assets:
Marketable equity securities	825	635
Fair Value, Measurements, Recurring | Level 1 | Deferred compensation plan
Assets:
Marketable equity securities	139	116
Liabilities:
Deferred compensation plan	139	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	20,420	25,539
Liabilities:
Total	6	93
Fair Value, Measurements, Recurring | Level 2 | U.S. treasury securities
Assets:
Available-for-sale debt securities	0	0
Fair Value, Measurements, Recurring | Level 2 | Certificates of deposit
Assets:
Available-for-sale debt securities	4,392	5,131
Fair Value, Measurements, Recurring | Level 2 | U.S. government agencies securities
Assets:
Available-for-sale debt securities	932	926
Fair Value, Measurements, Recurring | Level 2 | Non-U.S. government securities
Assets:
Available-for-sale debt securities	260	664
Fair Value, Measurements, Recurring | Level 2 | Corporate debt securities
Assets:
Available-for-sale debt securities	12,757	14,747
Fair Value, Measurements, Recurring | Level 2 | Residential mortgage and asset-backed securities
Assets:
Available-for-sale debt securities	2,037	4,058
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	42	13
Liabilities:
Foreign currency derivative contracts	6	93
Fair Value, Measurements, Recurring | Level 3
Assets:
Total	0	0
Liabilities:
Total	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 | Certificates of deposit
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 | Non-U.S. government securities
Assets:
Available-for-sale debt securities	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 | Residential mortgage and asset-backed 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

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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.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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