Equity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jan. 01, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, beginning balance (in shares)			3,577,103,522
Equity, beginning balance	$ 27,670	$ 33,901	$ 26,882	$ 34,569
Net income attributable to Merck & Co., Inc.	2,670	1,707	5,585	2,443
Adoption of new accounting standards					$ 48
Other comprehensive income (loss), net of taxes	(16)	(62)	183	62
Cash dividends declared on common stock	(1,440)	(1,291)	(2,869)	(2,592)
Treasury stock shares purchased	(1,235)	(1,596)	(2,325)	(2,162)
Share-based compensation plans and other	117	103	360	394
Net income (loss) attributable to noncontrolling interests	(26)	9	$ (79)	14
Distributions attributable to noncontrolling interests		(5)		(10)
Other changes in noncontrolling ownership interests	$ (3)
Shares, ending balance (in shares)	3,577,103,522		3,577,103,522
Equity, ending balance	$ 27,737	$ 32,766	$ 27,737	$ 32,766
Cash dividends declared on common stock (in dollars per share)	$ 0.55	$ 0.48	$ 1.10	$ 0.96
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, beginning balance (in shares)	3,577,000,000	3,577,000,000	3,577,000,000	3,577,000,000
Equity, beginning balance	$ 1,788	$ 1,788	$ 1,788	$ 1,788
Shares, ending balance (in shares)	3,577,000,000	3,577,000,000	3,577,000,000	3,577,000,000
Equity, ending balance	$ 1,788	$ 1,788	$ 1,788	$ 1,788
Other Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity, beginning balance	38,768	39,874	38,808	39,902
Treasury stock shares purchased	1,000		1,000
Share-based compensation plans and other	(284)	(133)	(324)	(161)
Equity, ending balance	39,484	39,741	39,484	39,741
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity, beginning balance	44,065	41,107	42,579	41,350
Net income attributable to Merck & Co., Inc.	2,670	1,707	5,585	2,443
Adoption of new accounting standards					322
Cash dividends declared on common stock	(1,440)	(1,291)	(2,869)	(2,592)
Equity, ending balance	45,295	41,523	45,295	41,523
Accumulated Other Comprehensive Loss
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity, beginning balance	(5,346)	(5,060)	(5,545)	(4,910)
Adoption of new accounting standards					$ (274)
Other comprehensive income (loss), net of taxes	(16)	(62)	183	62
Equity, ending balance	$ (5,362)	$ (5,122)	$ (5,362)	$ (5,122)
Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, beginning balance (in shares)	994,000,000	885,000,000	985,000,000	880,000,000
Equity, beginning balance	$ (51,736)	$ (44,041)	$ (50,929)	$ (43,794)
Treasury stock shares purchased (in shares)	24,000,000	27,000,000	38,000,000	37,000,000
Treasury stock shares purchased	$ (2,235)	$ (1,596)	$ (3,325)	$ (2,162)
Share-based compensation plans and other (in shares)	(8,000,000)	(5,000,000)	(13,000,000)	(10,000,000)
Share-based compensation plans and other	$ 401	$ 236	$ 684	$ 555
Shares, ending balance (in shares)	1,010,000,000	907,000,000	1,010,000,000	907,000,000
Equity, ending balance	$ (53,570)	$ (45,401)	$ (53,570)	$ (45,401)
Non- Controlling Interests
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity, beginning balance	131	233	181	233
Net income (loss) attributable to noncontrolling interests	(26)	9	(79)	14
Distributions attributable to noncontrolling interests		(5)		(10)
Other changes in noncontrolling ownership interests	(3)
Equity, ending balance	$ 102	$ 237	$ 102	$ 237

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