Commitments and Contingencies (Details) - EUR (€) € in Millions	3 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Third Parties for Pharmaceutical Manufacturing and Research Activities
Long-term Purchase Commitment [Line Items]
Termination notice term	30 days
Oxford BioMedica (UK) Ltd.
Long-term Purchase Commitment [Line Items]
Termination notice term, prior to first commercial sale of product	2 months
The University of Massachusetts Medical School
Long-term Purchase Commitment [Line Items]
Termination notice term	90 days
Termination notice term, other party's uncured material breach	60 days
cGMP Grade Viral Vector Production
Long-term Purchase Commitment [Line Items]
Contractual obligation		€ 0.9