ORGANIZATION AND DESCRIPTION OF BUSINESS (Details)	6 Months Ended
Jun. 30, 2019 agreement
ORGANIZATION AND DESCRIPTION OF BUSINESS
Number of product development programs for which entity has obtained exclusive rights	2

Number of product development programs for which entity has obtained exclusive rights.

No definition available.

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Namespace Prefix:	ymab_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration