Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Net income attributable to Merck & Co., Inc.	$ 3,002	$ 2,670	$ 6,221	$ 5,585
Average common shares outstanding	2,527	2,574	2,531	2,579
Common shares issuable (1)	9	14	11	17
Average common shares outstanding assuming dilution	2,536	2,588	2,542	2,596
Basic earnings per common share attributable to Merck & Co., Inc. common shareholders	$ 1.19	$ 1.04	$ 2.46	$ 2.17
Earnings per common share assuming dilution attributable to Merck & Co., Inc. common shareholders	$ 1.18	$ 1.03	$ 2.45	$ 2.15
Common shares issuable under share-based compensation plans excluded from diluted earnings per common share because the effect would have been antidilutive (in shares)	8	3	4	4