Equity - Rollforward Reconciliations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, beginning balance (in shares)			3,577,103,522
Equity, beginning balance	$ 26,300	$ 27,670	$ 26,001	$ 26,882
Net income attributable to Merck & Co., Inc.	3,002	2,670	6,221	5,585
Other comprehensive income (loss), net of taxes	(2)	(16)	(200)	183
Cash dividends declared on common stock	(1,550)	(1,440)	(3,099)	(2,869)
Treasury stock shares purchased		(1,235)	(1,281)	(2,325)
Share-based compensation plans and other	(13)	117	94	360
Net income (loss) attributable to noncontrolling interests	8	(26)	$ 8	(79)
Noncontrolling interest, changes in ownership interest, other		(3)
Distributions attributable to noncontrolling interests	$ (1)
Shares, ending balance (in shares)	3,577,103,522		3,577,103,522
Equity, ending balance	$ 27,744	$ 27,737	$ 27,744	$ 27,737
Cash dividends declared on common stock (in dollars per share)	$ 0.61	$ 0.55	$ 1.22	$ 1.10
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	39,697	38,768	39,660	38,808
Treasury stock shares purchased		1,000		1,000
Share-based compensation plans and other	(324)	(284)	(287)	(324)
Equity, ending balance	39,373	39,484	39,373	39,484
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity, beginning balance	48,272	44,065	46,602	42,579
Net income attributable to Merck & Co., Inc.	3,002	2,670	6,221	5,585
Cash dividends declared on common stock	(1,550)	(1,440)	(3,099)	(2,869)
Equity, ending balance	49,724	45,295	49,724	45,295
Accumulated Other Comprehensive Loss
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity, beginning balance	(6,391)	(5,346)	(6,193)	(5,545)
Other comprehensive income (loss), net of taxes	(2)	(16)	(200)	183
Equity, ending balance	$ (6,393)	$ (5,362)	$ (6,393)	$ (5,362)
Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, beginning balance (in shares)	1,053,000,000	994,000,000	1,038,000,000	985,000,000
Equity, beginning balance	$ (57,161)	$ (51,736)	$ (55,950)	$ (50,929)
Treasury stock shares purchased (in shares)		24,000,000	16,000,000	38,000,000
Treasury stock shares purchased		$ (2,235)	$ (1,281)	$ (3,325)
Share-based compensation plans and other (in shares)	(5,000,000)	(8,000,000)	(6,000,000)	(13,000,000)
Share-based compensation plans and other	$ 311	$ 401	$ 381	$ 684
Shares, ending balance (in shares)	1,048,000,000	1,010,000,000	1,048,000,000	1,010,000,000
Equity, ending balance	$ (56,850)	$ (53,570)	$ (56,850)	$ (53,570)
Non- controlling Interests
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity, beginning balance	95	131	94	181
Net income (loss) attributable to noncontrolling interests	8	(26)	8	(79)
Noncontrolling interest, changes in ownership interest, other		(3)
Distributions attributable to noncontrolling interests	(1)
Equity, ending balance	$ 102	$ 102	$ 102	$ 102