Research Collaboration and License Agreement (Details) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2014 USD ($) payment
Research Collaboration and License Agreement
Number of development milestone payments \| payment					3
Aggregate development milestone payment \| $	$ 4.0	$ 0.0	$ 4.0	$ 0.0	$ 1.0

The aggregate amount of development milestone payment made by the entity.

No definition available.

Name:	ptgx_AggregateAmountOfDevelopmentMilestonePayment
Namespace Prefix:	ptgx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration