| Cover | Oct. 29, 2019 |
| Schedule of Capitalization, Equity [Line Items] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 29, 2019 |
| Entity File Number | 1-6571 |
| Entity Registrant Name | Merck & Co., Inc. |
| Entity Central Index Key | 0000310158 |
| Entity Tax Identification Number | 22-1918501 |
| Entity Incorporation, State or Country Code | NJ |
| 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 |
| City Area Code | 908 |
| Local Phone Number | 740-4000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Common Stock | |
| Schedule of Capitalization, Equity [Line Items] | |
| Title of 12(b) Security | Common Stock ($0.50 par value) |
| Trading Symbol | MRK |
| Security Exchange Name | NYSE |
| 1.125% Notes due 2021 | |
| Schedule of Capitalization, Equity [Line Items] | |
| Title of 12(b) Security | 1.125% Notes due 2021 |
| Trading Symbol | MRK/21 |
| Security Exchange Name | NYSE |
| 0.500% Notes due 2024 | |
| Schedule of Capitalization, Equity [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 | |
| Schedule of Capitalization, Equity [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 | |
| Schedule of Capitalization, Equity [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 | |
| Schedule of Capitalization, Equity [Line Items] | |
| Title of 12(b) Security | 1.375% Notes due 2036 |
| Trading Symbol | MRK 36A |
| Security Exchange Name | NYSE |
