Equity - Rollforward Reconciliations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, beginning balance (in shares)			3,577,103,522
Equity, beginning balance	$ 27,744	$ 27,737	$ 26,001	$ 26,882
Net income attributable to Merck & Co., Inc.	2,941	1,901	9,161	7,487
Other comprehensive income (loss), net of taxes	10	(28)	(190)	155
Cash dividends declared on common stock	(1,558)	(1,392)	(4,656)	(4,262)
Treasury stock shares purchased		(1,405)	(1,281)	(3,730)
Share-based compensation plans and other	151	127	245	487
Net income (loss) attributable to noncontrolling interests	4	6	12	(73)
Distributions attributable to noncontrolling interests	$ (22)	(21)	$ (22)	(21)
Shares, ending balance (in shares)	3,577,103,522		3,577,103,522
Equity, ending balance	$ 29,270	$ 26,925	$ 29,270	$ 26,925
Cash dividends declared on common stock (in dollars per share)	$ 0.61	$ 0.55	$ 1.83	$ 1.65
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	39,373	39,484	39,660	38,808
Treasury stock shares purchased				1,000
Share-based compensation plans and other	116	77	(171)	(247)
Equity, ending balance	39,489	39,561	39,489	39,561
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity, beginning balance	49,724	45,295	46,602	42,579
Net income attributable to Merck & Co., Inc.	2,941	1,901	9,161	7,487
Cash dividends declared on common stock	(1,558)	(1,392)	(4,656)	(4,262)
Equity, ending balance	51,107	45,804	51,107	45,804
Accumulated Other Comprehensive Loss
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity, beginning balance	(6,393)	(5,362)	(6,193)	(5,545)
Other comprehensive income (loss), net of taxes	10	(28)	(190)	155
Equity, ending balance	$ (6,383)	$ (5,390)	$ (6,383)	$ (5,390)
Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, beginning balance (in shares)	1,048,000,000	1,010,000,000	1,038,000,000	985,000,000
Equity, beginning balance	$ (56,850)	$ (53,570)	$ (55,950)	$ (50,929)
Treasury stock shares purchased (in shares)		17,000,000	16,000,000	54,000,000
Treasury stock shares purchased		$ (1,405)	$ (1,281)	$ (4,730)
Share-based compensation plans and other (in shares)	(1,000,000)	(1,000,000)	(7,000,000)	(13,000,000)
Share-based compensation plans and other	$ 35	$ 50	$ 416	$ 734
Shares, ending balance (in shares)	1,047,000,000	1,026,000,000	1,047,000,000	1,026,000,000
Equity, ending balance	$ (56,815)	$ (54,925)	$ (56,815)	$ (54,925)
Non- controlling Interests
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity, beginning balance	102	102	94	181
Net income (loss) attributable to noncontrolling interests	4	6	12	(73)
Distributions attributable to noncontrolling interests	(22)	(21)	(22)	(21)
Equity, ending balance	$ 84	$ 87	$ 84	$ 87