Earnings Per Share - Calculations of Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net Income from Continuing Operations Attributable to Merck & Co., Inc.	$ 4,567	$ 2,324	$ 8,525	$ 7,136
Income from Discontinued Operations, Net of Taxes and Amounts Attributable to Noncontrolling Interests	0	617	766	2,025
Net income attributable to Merck & Co., Inc., basic	4,567	2,941	9,291	9,161
Net income attributable to Merck & Co., Inc., diluted	$ 4,567	$ 2,941	$ 9,291	$ 9,161
Average common shares outstanding (in shares)	2,530	2,529	2,531	2,530
Common shares issuable (in shares)	6	9	8	11
Average common shares outstanding assuming dilution (in shares)	2,536	2,538	2,539	2,541
Basic Earnings per Common Share Attributable to Merck & Co., Inc. Common Shareholders:
Income from Continuing Operations (in dollars per share)	$ 1.81	$ 0.92	$ 3.37	$ 2.82
Income from Discontinued Operations (in dollars per share)	0	0.24	0.30	0.80
Net Income (in dollars per share)	1.81	1.16	3.67	3.62
Earnings per Common Share Assuming Dilution Attributable to Merck & Co., Inc. Common Shareholders:
Income from Continuing Operations (in dollars per share)	1.80	0.92	3.36	2.81
Income from Discontinued Operations (in dollars per share)	0	0.24	0.30	0.80
Net Income (in dollars per share)	$ 1.80	$ 1.16	$ 3.66	$ 3.61
Antidilutive shares (in shares)	8	5	10	5

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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- Definition

Per basic share amount, after tax, of income (loss) from the day-to-day business activities of the discontinued operation.

+ References

No definition available.