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License and Collaboration Agreement (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended		12 Months Ended
	May 07, 2019	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jul. 13, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payments and fees		$ 23,792				$ 23,792
Revenue recognized					$ 14,100	1,600	$ 23,500
Costs to obtain or fulfill the contract that were capitalized		0			$ 0	0	0
License and Collaborative Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development cost					20.00%
Second-Generation Analog, Phase 3 CD Clinical Trial Primary Clinical Endpoint [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development cost					80.00%
Second Generation Analog Phase 2 Cd Clinical Trial Primary Clinical
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development cost					80.00%
Phase 2 clinical trial \| Additional services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue							100
First Amendment \| Development, regulatory and sales milestone payments
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Eligible payment receivable	$ 1,000,000
First Amendment \| Development, regulatory and sales milestone payments \| Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentages on net product sales	10.00%
Janssen Biotech, Inc. \| License and Collaborative Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue					$ 27,100
Revenue recognized					28,600	200	30,900
Janssen Biotech, Inc. \| Additional services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue					$ 1,500	0
Janssen Biotech, Inc. \| Second-Generation Analog, Phase 3 CD Clinical Trial Primary Clinical Endpoint [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development cost					20.00%
Janssen Biotech, Inc. \| License and Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Increase (decrease) in transaction price					$ (14,300)	37,900
Transaction price		112,900			98,600	112,900	60,700
Revenue							$ 30,800
Janssen Biotech, Inc. \| License and Collaboration Agreement \| Upfront cash payment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payments and fees								$ 50,000
Janssen Biotech, Inc. \| First, second-generation compound \| Milestone payment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount payable upon the effectiveness of the First Amendment					$ 7,500
Janssen Biotech, Inc. \| First, second-generation compound \| Second-Generation Analog, Phase 3 CD Clinical Trial Primary Clinical Endpoint [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research program cost					100.00%
Janssen Biotech, Inc. \| Second and third, second-generation compounds \| Second-Generation Analog, Phase 3 CD Clinical Trial Primary Clinical Endpoint [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research program cost					50.00%
Janssen Biotech, Inc. \| Thereafter \| Second-Generation Analog, Phase 3 CD Clinical Trial Primary Clinical Endpoint [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research program cost					100.00%
Janssen Biotech, Inc. \| Second-Generation Phase 2b Milestone
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount payable upon the effectiveness of the First Amendment		$ 5,000
Janssen Biotech, Inc. \| Second-Generation Phase 2b Milestone \| Milestone payment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized					$ 6,800
Janssen Biotech, Inc. \| Phase 2 clinical trial \| License and Collaborative Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue					17,900
Janssen Biotech, Inc. \| First Amendment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Increase (decrease) in transaction price						(9,400)
Revenue recognized					5,000
Janssen Biotech, Inc. \| First Amendment \| License and Collaborative Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Eligible amount received			$ 25,000
Revenue recognized			25,000
Revenue following contract modification						$ 8,000
Revenue recognized from performance obligations in previous periods				$ 1,600
Janssen Biotech, Inc. \| First Amendment \| PTG-200, Phase 3 CD Clinical Trial Primary Clinical Endpoint
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Eligible payment receivable	$ 100,000
Janssen Biotech, Inc. \| First Amendment \| Second-Generation Analog, Phase 3 CD Clinical Trial Primary Clinical Endpoint [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Eligible payment receivable	$ 115,000
Janssen Biotech, Inc. \| First Amendment \| Second-Generation Phase 2b Milestone Payment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount payable upon the effectiveness of the First Amendment					50,000
Janssen Biotech, Inc. \| Amended First Opt-in \| Upfront cash payment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount payable upon the effectiveness of the First Amendment					50,000
Janssen Biotech, Inc. \| Amended Second Opt-in \| Upfront cash payment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount payable upon the effectiveness of the First Amendment					50,000
Janssen Biotech, Inc. \| Estimated variable consideration \| License and Collaborative Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount payable upon the effectiveness of the First Amendment					$ 7,500
Janssen Biotech, Inc. \| Pro Forma \| License and Collaborative Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount payable upon the effectiveness of the First Amendment			25,000
Janssen Biotech, Inc. \| Pro Forma \| Milestone payment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount payable upon the effectiveness of the First Amendment			$ 25,000