Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Assets
Investments	$ 16,234	$ 20,179
Derivative assets	221	995
Liabilities
Contingent consideration	945	891	$ 894	$ 590
Derivative liabilities	227	134
Commercial paper
Assets
Investments	695	4,330
Corporate notes and bonds
Assets
Investments	10,259	10,577
U.S. government and agency securities
Assets
Investments	1,924	2,232
Asset-backed securities
Assets
Investments	$ 1,473	1,376
Liabilities
Primary weighted average life of collateral	5 years
Foreign government bonds
Assets
Investments	$ 640	519
Mortgage-backed securities
Assets
Investments	704	796
Equity securities
Assets
Investments	539	349
Fair Value, Measurements, Recurring
Assets
Investments	15,500	19,242
Other assets	734	937
Derivative assets	221	995
Total assets	16,455	21,174
Liabilities
Contingent consideration	945	891
Derivative liabilities	227	134
Total liabilities	1,172	1,025
Fair Value, Measurements, Recurring | Purchased currency options
Assets
Derivative assets	119	644
Liabilities
Derivative liabilities	0	12
Fair Value, Measurements, Recurring | Foreign exchange contracts
Assets
Derivative assets	87	331
Liabilities
Derivative liabilities	201	93
Fair Value, Measurements, Recurring | Interest rate swap contracts
Assets
Derivative assets	15	20
Liabilities
Derivative liabilities	26	29
Fair Value, Measurements, Recurring | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets
Investments	436	230
Other assets	171	148
Derivative assets	0	0
Total assets	607	378
Liabilities
Contingent consideration	0	0
Derivative liabilities	0	0
Total liabilities	0	0
Fair Value, Measurements, Recurring | Quoted Prices In Active Markets for Identical Assets (Level 1) | Purchased currency options
Assets
Derivative assets	0	0
Liabilities
Derivative liabilities	0	0
Fair Value, Measurements, Recurring | Quoted Prices In Active Markets for Identical Assets (Level 1) | Foreign exchange contracts
Assets
Derivative assets	0	0
Liabilities
Derivative liabilities	0	0
Fair Value, Measurements, Recurring | Quoted Prices In Active Markets for Identical Assets (Level 1) | Interest rate swap contracts
Assets
Derivative assets	0	0
Liabilities
Derivative liabilities	0	0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Assets
Investments	15,064	19,012
Other assets	563	789
Derivative assets	221	995
Total assets	15,848	20,796
Liabilities
Contingent consideration	0	0
Derivative liabilities	227	134
Total liabilities	227	134
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Purchased currency options
Assets
Derivative assets	119	644
Liabilities
Derivative liabilities	0	12
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Foreign exchange contracts
Assets
Derivative assets	87	331
Liabilities
Derivative liabilities	201	93
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Interest rate swap contracts
Assets
Derivative assets	15	20
Liabilities
Derivative liabilities	26	29
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Assets
Investments	0	0
Other assets	0	0
Derivative assets	0	0
Total assets	0	0
Liabilities
Contingent consideration	945	891
Derivative liabilities	0	0
Total liabilities	945	891
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Purchased currency options
Assets
Derivative assets	0	0
Liabilities
Derivative liabilities	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Foreign exchange contracts
Assets
Derivative assets	0	0
Liabilities
Derivative liabilities	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Interest rate swap contracts
Assets
Derivative assets	0	0
Liabilities
Derivative liabilities	0	0
Fair Value, Measurements, Recurring | Commercial paper
Assets
Investments	695	4,330
Fair Value, Measurements, Recurring | Commercial paper | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets
Investments	0	0
Fair Value, Measurements, Recurring | Commercial paper | Significant Other Observable Inputs (Level 2)
Assets
Investments	695	4,330
Fair Value, Measurements, Recurring | Commercial paper | Significant Unobservable Inputs (Level 3)
Assets
Investments	0	0
Fair Value, Measurements, Recurring | Corporate notes and bonds
Assets
Investments	10,110	10,389
Other assets	149	188
Fair Value, Measurements, Recurring | Corporate notes and bonds | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets
Investments	0	0
Other assets	0	0
Fair Value, Measurements, Recurring | Corporate notes and bonds | Significant Other Observable Inputs (Level 2)
Assets
Investments	10,110	10,389
Other assets	149	188
Fair Value, Measurements, Recurring | Corporate notes and bonds | Significant Unobservable Inputs (Level 3)
Assets
Investments	0	0
Other assets	0	0
Fair Value, Measurements, Recurring | U.S. government and agency securities
Assets
Investments	1,690	1,919
Other assets	234	313
Fair Value, Measurements, Recurring | U.S. government and agency securities | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets
Investments	68	29
Other assets	0	0
Fair Value, Measurements, Recurring | U.S. government and agency securities | Significant Other Observable Inputs (Level 2)
Assets
Investments	1,622	1,890
Other assets	234	313
Fair Value, Measurements, Recurring | U.S. government and agency securities | Significant Unobservable Inputs (Level 3)
Assets
Investments	0	0
Other assets	0	0
Fair Value, Measurements, Recurring | Asset-backed securities
Assets
Investments	1,395	1,257
Other assets	78	119
Fair Value, Measurements, Recurring | Asset-backed securities | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets
Investments	0	0
Other assets	0	0
Fair Value, Measurements, Recurring | Asset-backed securities | Significant Other Observable Inputs (Level 2)
Assets
Investments	1,395	1,257
Other assets	78	119
Fair Value, Measurements, Recurring | Asset-backed securities | Significant Unobservable Inputs (Level 3)
Assets
Investments	0	0
Other assets	0	0
Fair Value, Measurements, Recurring | Foreign government bonds
Assets
Investments	639	518
Other assets	1	1
Fair Value, Measurements, Recurring | Foreign government bonds | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets
Investments	0	0
Other assets	0	0
Fair Value, Measurements, Recurring | Foreign government bonds | Significant Other Observable Inputs (Level 2)
Assets
Investments	639	518
Other assets	1	1
Fair Value, Measurements, Recurring | Foreign government bonds | Significant Unobservable Inputs (Level 3)
Assets
Investments	0	0
Other assets	0	0
Fair Value, Measurements, Recurring | Mortgage-backed securities
Assets
Investments	603	628
Other assets	101	168
Fair Value, Measurements, Recurring | Mortgage-backed securities | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets
Investments	0	0
Other assets	0	0
Fair Value, Measurements, Recurring | Mortgage-backed securities | Significant Other Observable Inputs (Level 2)
Assets
Investments	603	628
Other assets	101	168
Fair Value, Measurements, Recurring | Mortgage-backed securities | Significant Unobservable Inputs (Level 3)
Assets
Investments	0	0
Other assets	0	0
Fair Value, Measurements, Recurring | Equity securities
Assets
Investments	368	201
Other assets	171	148
Fair Value, Measurements, Recurring | Equity securities | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets
Investments	368	201
Other assets	171	148
Fair Value, Measurements, Recurring | Equity securities | Significant Other Observable Inputs (Level 2)
Assets
Investments	0	0
Other assets	0	0
Fair Value, Measurements, Recurring | Equity securities | Significant Unobservable Inputs (Level 3)
Assets
Investments	0	0
Other assets	$ 0	$ 0