Earnings Per Share - Calculations of (Loss) Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Earnings Per Share [Abstract]
Net (loss) income attributable to Merck & Co., Inc., basic	$ (5,975)	$ 3,944	$ (3,154)	$ 8,254
Net (loss) income attributable to Merck & Co., Inc., diluted	$ (5,975)	$ 3,944	$ (3,154)	$ 8,254
Average common shares outstanding (in shares)	2,539	2,531	2,539	2,529
Common shares issuable (in shares)	0	9	0	9
Average common shares outstanding assuming dilution (in shares)	2,539	2,540	2,539	2,538
Basic (Loss) Earnings per Common Share Attributable to Merck & Co., Inc. Common Shareholders (in dollars per share)	$ (2.35)	$ 1.56	$ (1.24)	$ 3.26
(Loss) Earnings per Common Share Attributable to Merck & Co., Inc. Common Shareholders (in dollars per share)	$ (2.35)	$ 1.55	$ (1.24)	$ 3.25