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Significant Accounting Policies - Additional Information (Detail) € in Millions | 6 Months Ended
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Jun. 30, 2019 EUR (€) Segment
Disclosure of significant accounting policies [abstract] |
Number of reportable segment | Segment | 1
Cash and cash equivalents, maximum original maturities | 3 months
Short-term investments | € | € 49.0

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- Definition A classification of cash equivalents representing short-term investments. [Refer: Cash equivalents] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 7 -IssueDate 2018-01-01 -Paragraph 45 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=7&date=2018-03-01&anchor=para_45&doctype=Standard -URIDate 2018-03-16 + Details
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Balance Type: | debit
Period Type: | instant

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- Definition Cash And Cash Equivalents Maximum Maturity Period + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition Disclosure of significant accounting policies. + References No definition available. + Details
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- Definition Number of reportable segments. + References No definition available. + Details
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Namespace Prefix: | mrus_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration