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Cover Page - USD ($)
 12 Months Ended
Dec. 31, 2021
Jan. 31, 2022
Jun. 30, 2021
Document Information [Line Items]      
Document Type 10-K    
Document Annual Report true    
Document Transition Report false    
Entity File Number 1-6571    
Entity Registrant Name Merck & Co., Inc.    
City Area Code 908    
Local Phone Number 740-4000    
Entity Incorporation, State or Country Code NJ    
Entity Tax Identification Number 22-1918501    
Entity Address, Address Line One 2000 Galloping Hill Road    
Entity Address, City or Town Kenilworth    
Entity Address, State or Province NJ    
Entity Address, Postal Zip Code 07033    
Title of 12(b) Security Common Stock ($0.50 par value)    
Trading Symbol MRK    
Security Exchange Name NYSE    
Entity Common Stock, Shares Outstanding   2,527,733,606  
Entity Public Float     $ 196,870,000,000
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Amendment Flag false    
Document Period End Date Dec. 31, 2021    
Document Fiscal Year Focus 2021    
Document Fiscal Period Focus FY    
Entity Central Index Key 0000310158    
Current Fiscal Year End Date --12-31    
Documents Incorporated by Reference
Proxy Statement for the Annual Meeting of Shareholders to be held May 24, 2022, to be filed with the
Securities and Exchange Commission within 120 days after the close of the fiscal year covered by this report
    
0.500% Notes due 2024      
Document Information [Line Items]      
Title of 12(b) Security 0.500% Notes due 2024    
Trading Symbol MRK 24    
Security Exchange Name NYSE    
1.875% Notes due 2026      
Document Information [Line Items]      
Title of 12(b) Security 1.875% Notes due 2026    
Trading Symbol MRK/26    
Security Exchange Name NYSE    
2.500% Notes due 2034      
Document Information [Line Items]      
Title of 12(b) Security 2.500% Notes due 2034    
Trading Symbol MRK/34    
Security Exchange Name NYSE    
1.375% Notes due 2036      
Document Information [Line Items]      
Title of 12(b) Security 1.375% Notes due 2036    
Trading Symbol MRK 36A    
Security Exchange Name NYSE