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Earnings Per Share - Calculations of Earnings Per Share (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Earnings Per Share [Abstract]      
Net Income Attributable to Merck & Co., Inc. $ 2,394 $ 3,920 $ 4,442
Average common shares outstanding, in shares 2,730 2,766 2,816
Common shares issuable, in shares 18 21 25
Average common shares outstanding assuming dilution, in shares 2,748 2,787 2,841
Basic earnings per common share attributable to Merck & Co., Inc. common shareholders, in dollars per share $ 0.88 $ 1.42 $ 1.58
Earnings per common share assuming dilution attributable to Merck & Co., Inc. common shareholders, in dollars per share $ 0.87 $ 1.41 $ 1.56