Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share [Abstract]
Net income attributable to Merck & Co., Inc.	$ 2,941	$ 1,901	$ 9,161	$ 7,487
Average common shares outstanding	2,529	2,558	2,530	2,572
Common shares issuable (1)	9	14	11	15
Average common shares outstanding assuming dilution	2,538	2,572	2,541	2,587
Basic earnings per common share attributable to Merck & Co., Inc. common shareholders	$ 1.16	$ 0.74	$ 3.62	$ 2.91
Earnings per common share assuming dilution attributable to Merck & Co., Inc. common shareholders	$ 1.16	$ 0.74	$ 3.61	$ 2.89
Common shares issuable under share-based compensation plans excluded from diluted earnings per common share because the effect would have been antidilutive (in shares)	5	3	5	2