Fair Value Measurements Summary of assets and liabilities measured at fair value (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Assets:
Available-for-sale debt securities	$ 24,515	$ 24,164
Total	5,045	6,310
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement
Assets:
Total	29,637	30,552
Liabilities:
Total	152	125
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | U.S. treasury securities
Assets:
Available-for-sale debt securities	4,782	3,969
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Certificates of deposit
Assets:
Available-for-sale debt securities	4,211	4,361
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | U.S. government agencies securities
Assets:
Available-for-sale debt securities	1,444	938
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Municipal debt securities
Assets:
Available-for-sale debt securities	25	0
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Non-U.S. government securities
Assets:
Available-for-sale debt securities	452	305
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Corporate debt securities
Assets:
Available-for-sale debt securities	12,565	13,067
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Residential mortgage and asset-backed securities
Assets:
Available-for-sale debt securities	1,036	1,524
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Money market funds
Assets:
Marketable equity securities	3,778	5,305
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Publicly traded equity securities
Assets:
Marketable equity securities	1,117	881
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Deferred compensation plan
Assets:
Marketable equity securities	150	124
Liabilities:
Deferred compensation plan	150	124
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement | Foreign currency derivative contracts
Assets:
Foreign currency derivative contracts	77	78
Liabilities:
Foreign currency derivative contracts	2	1
Fair Value, Measurements, Recurring | Level 1
Assets:
Total	9,809	10,279
Liabilities:
Total	150	124
Fair Value, Measurements, Recurring | Level 1 | U.S. treasury securities
Assets:
Available-for-sale debt securities	4,782	3,969
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 | Municipal debt 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	3,778	5,305
Fair Value, Measurements, Recurring | Level 1 | Publicly traded equity securities
Assets:
Marketable equity securities	1,099	881
Fair Value, Measurements, Recurring | Level 1 | Deferred compensation plan
Assets:
Marketable equity securities	150	124
Liabilities:
Deferred compensation plan	150	124
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	19,828	20,273
Liabilities:
Total	2	1
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	4,211	4,361
Fair Value, Measurements, Recurring | Level 2 | U.S. government agencies securities
Assets:
Available-for-sale debt securities	1,444	938
Fair Value, Measurements, Recurring | Level 2 | Municipal debt securities
Assets:
Available-for-sale debt securities	25	0
Fair Value, Measurements, Recurring | Level 2 | Non-U.S. government securities
Assets:
Available-for-sale debt securities	452	305
Fair Value, Measurements, Recurring | Level 2 | Corporate debt securities
Assets:
Available-for-sale debt securities	12,565	13,067
Fair Value, Measurements, Recurring | Level 2 | Residential mortgage and asset-backed securities
Assets:
Available-for-sale debt securities	1,036	1,524
Fair Value, Measurements, Recurring | Level 2 | Money market funds
Assets:
Marketable equity securities	0	0
Fair Value, Measurements, Recurring | Level 2 | Publicly traded equity securities
Assets:
Marketable equity securities	18	0
Fair Value, Measurements, Recurring | Level 2 | Deferred compensation plan
Assets:
Marketable equity securities		0
Liabilities:
Deferred compensation plan	0	0
Fair Value, Measurements, Recurring | Level 2 | Foreign currency derivative contracts
Assets:
Foreign currency derivative contracts	77	78
Liabilities:
Foreign currency derivative contracts	2	1
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 | Municipal debt 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 | Publicly traded 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