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Operating Leases - Additional Information (Details)
€ in Millions
1 Months Ended 12 Months Ended
Jul. 31, 2019
EUR (€)
RenewalTerm
Mar. 31, 2019
Dec. 31, 2021
Lessee Lease Description [Line Items]      
Extended lease term     1 year 6 months
Operating lease, description     Merus N.V. has non-cancellable operating leases for its corporate headquarters in Utrecht, the Netherlands. During the year ended December 31, 2021, the Company signed a lease amendment (the “Amendment”), which extended the lease term by approximately 1.5 years ending at the end of March 2023. The Amendment did not include additional right-of-use other than the extended lease term.
Operating lease, expiration period     March 2023
Additional renewal term     no
Scenario Plan [Member] | Kadans Science Partner XII BV | Office Building      
Lessee Lease Description [Line Items]      
Extended lease term 10 years    
Area of property available for operating lease | m² 5,070    
Operating lease, renewal term 5 years    
Operating lease, number of renewal term | RenewalTerm 2    
Operating lease, termination term 20 years    
Operating lease, estimated rent expense | € € 1.3    
Operating lease, rent adjustment description The rent amount is subject to adjustment based on the consumer price index (the “CPI”) beginning on January 1, 2019 through the completion date and then annually thereafter, subject to certain limitations if the CPI is greater than 3.0%.    
Scenario Plan [Member] | Kadans Science Partner XII BV | Minimum | Office Building      
Lessee Lease Description [Line Items]      
Consumer price Index 3.00%    
Merus US Inc      
Lessee Lease Description [Line Items]      
Extended lease term   7 years  
Operating lease, description   Merus US, Inc. entered into a non-cancellable operating lease agreement for office space in Cambridge, Massachusetts. The lease commenced in the second quarter of 2019 and has a term of seven years, and may be extended for another five years. Given the Company’s current plans, the renewal term has not been included in the estimate of the lease term.  
Operating lease, extended term of agreement   5 years  
Operating lease, option to extend   true  
Operating lease, options to extend description   may be extended for another five years.