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Fair Value Measurements - Summary of assets and liabilities measured at fair value (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Assets:
Available-for-sale debt securities	$ 22,561	$ 29,587
Total	7,854	5,465
Fair Value, Measurements, Recurring | Level 1
Assets:
Total	11,245	9,526
Liabilities:
Total	131	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	3,391	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	7,060	4,714
Fair Value, Measurements, Recurring | Level 1 | Equity securities
Assets:
Marketable equity securities	663	635
Fair Value, Measurements, Recurring | Level 1 | Deferred compensation plan
Assets:
Marketable equity securities	131	116
Liabilities:
Deferred compensation plan	131	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	19,223	25,539
Liabilities:
Total	11	93
Fair Value, Measurements, Recurring | Level 2 | Corporate debt securities
Assets:
Available-for-sale debt securities	12,092	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	2,657	4,058
Fair Value, Measurements, Recurring | Level 2 | U.S. government agencies securities
Assets:
Available-for-sale debt securities	872	926
Fair Value, Measurements, Recurring | Level 2 | Certificates of deposit
Assets:
Available-for-sale debt securities	3,145	5,131
Fair Value, Measurements, Recurring | Level 2 | Non-U.S. government securities
Assets:
Available-for-sale debt securities	404	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	53	13
Liabilities:
Foreign currency derivative contracts	11	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	30,468	35,065
Liabilities:
Total	142	209
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Corporate debt securities
Assets:
Available-for-sale debt securities	12,092	14,747
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | U.S. treasury securities
Assets:
Available-for-sale debt securities	3,391	4,061
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Residential mortgage and asset-backed securities
Assets:
Available-for-sale debt securities	2,657	4,058
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | U.S. government agencies securities
Assets:
Available-for-sale debt securities	872	926
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Certificates of deposit
Assets:
Available-for-sale debt securities	3,145	5,131
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Non-U.S. government securities
Assets:
Available-for-sale debt securities	404	664
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Money market funds
Assets:
Marketable equity securities	7,060	4,714
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Equity securities
Assets:
Marketable equity securities	663	635
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Deferred compensation plan
Assets:
Marketable equity securities	131	116
Liabilities:
Deferred compensation plan	131	116
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Foreign currency derivative contracts
Assets:
Foreign currency derivative contracts	53	13
Liabilities:
Foreign currency derivative contracts	$ 11	$ 93

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