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Earnings Per Share (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Earnings Per Share [Abstract]    
Net income attributable to Merck & Co., Inc. $ 3,179 $ 3,219
Average common shares outstanding 2,531 2,533
Common shares issuable (1) 10 14
Average common shares outstanding assuming dilution 2,541 2,547
Basic earnings per common share attributable to Merck & Co., Inc. common shareholders $ 1.26 $ 1.27
Earnings per common share assuming dilution attributable to Merck & Co., Inc. common shareholders $ 1.25 $ 1.26
Common shares issuable under share-based compensation plans excluded from diluted earnings per common share because the effect would have been antidilutive (in shares) 14 2