| Document and Entity Information | Sep. 02, 2020 |
| Cover [Abstract] | |
| Entity Tax Identification Number | 00-0000000 |
| Amendment Flag | false |
| Entity Central Index Key | 0001651311 |
| Document Type | 8-K |
| Document Period End Date | Sep. 2, 2020 |
| Entity Registrant Name | MERUS N.V. |
| Entity Incorporation State Country Code | P7 |
| Entity File Number | 001-37773 |
| Entity Address, Address Line One | Yalelaan 62 |
| Entity Address, Postal Zip Code | 3584 CM |
| Entity Address, City or Town | Utrecht |
| Entity Address, Country | NL |
| City Area Code | +31 85 |
| Local Phone Number | 016 2500 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Shares, €0.09 nominal value per share |
| Trading Symbol | MRUS |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
