Earnings Per Share - Calculations of Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Earnings Per Share [Abstract]
Net Income Attributable to Merck & Co., Inc., basic	$ 4,745	$ 3,248	$ 1,591	$ 11,502
Net Income Attributable to Merck & Co., Inc., diluted	$ 4,745	$ 3,248	$ 1,591	$ 11,502
Average common shares outstanding (in shares)	2,537	2,533	2,538	2,531
Common shares issuable (in shares)	9	9	11	9
Average common shares outstanding assuming dilution (in shares)	2,546	2,542	2,549	2,540
Basic Earnings per Common Share Attributable to Merck & Co., Inc. Common Shareholders (in dollars per share)	$ 1.87	$ 1.28	$ 0.63	$ 4.55
Earnings per Common Share Attributable to Merck & Co., Inc. Common Shareholders (in dollars per share)	$ 1.86	$ 1.28	$ 0.62	$ 4.53