Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Earnings Per Share [Abstract]
Net Income from Continuing Operations Attributable to Merck & Co., Inc.	$ 1,213	$ 2,341	$ 3,958	$ 4,812
Income from Discontinued Operations, Net of Taxes and Amounts Attributable to Noncontrolling Interests	332	661	766	1,409
Net income attributable to Merck & Co., Inc.	$ 1,545	$ 3,002	$ 4,724	$ 6,221
Average common shares outstanding	2,533	2,527	2,532	2,531
Common shares issuable	7	9	8	11
Average common shares outstanding assuming dilution	2,540	2,536	2,540	2,542
Basic Earnings per Common Share Attributable to Merck & Co., Inc. Common Shareholders:
Income from Continuing Operations	$ 0.48	$ 0.93	$ 1.56	$ 1.90
Income from Discontinued Operations	0.13	0.26	0.30	0.56
Net Income	0.61	1.19	1.87	2.46
Earnings per Common Share Assuming Dilution Attributable to Merck & Co., Inc. Common Shareholders:
Income from Continuing Operations	0.48	0.92	1.56	1.89
Income from Discontinued Operations	0.13	0.26	0.30	0.55
Net Income	$ 0.61	$ 1.18	$ 1.86	$ 2.45
Common shares issuable under share-based compensation plans excluded from diluted earnings per common share because the effect would have been antidilutive (in shares)	12	8	11	4

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.