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Earnings Per Share - Calculations of Earnings Per Share (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Earnings Per Share [Abstract]    
Net income attributable to Merck & Co., Inc. $ 736 $ 1,551
Average common shares outstanding (in shares) 2,695 2,745
Common shares issuable (in shares) 15 21
Average common shares outstanding assuming dilution (in shares) 2,710 2,766
Basic earnings per common share attributable to Merck & Co., Inc. common shareholders (in dollars per share) $ 0.27 $ 0.56
Earnings per common share assuming dilution attributable to Merck & Co., Inc. common shareholders (in dollars per share) $ 0.27 $ 0.56