License and Collaboration Agreement (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	May 07, 2019	Dec. 31, 2018	Dec. 31, 2017	Jul. 13, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payments and fees	$ 22,200				$ 22,200
Revenue recognized	2,900			$ 5,900	4,500	$ 27,900
Deferred revenue - related party	16,478				16,478			$ 8,223
Receivable from collaboration partner and contract asset - related party	1,945				1,945			4,587
Payable to collaboration partner - related party	1,020				$ 1,020			1,061
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%
First Amendment | Development, regulatory and sales milestone payments
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount to be received							$ 1,000,000
Janssen Biotech, Inc. | License and Collaborative Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized	4,100		$ (1,600)	6,100		28,600
Increase (decrease) in transaction price					$ (9,400)
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]
Deferred revenue - related party	38,678			7,970	$ 38,678	7,970		8,223	$ 31,752
Payable to collaboration partner - related party	1,020			$ 764	1,020	764		$ 1,061
Transaction price recognized based on proportional performance		$ 109,200	$ 60,600		5,986	$ 27,929
Transaction price	110,800				$ 110,800
Development cost					80.00%
Increase (decrease) in transaction price	1,600	50,200
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 | 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. | Phase 2 clinical trial | License and Collaborative Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue					$ 18,400
Janssen Biotech, Inc. | First Opt-in Election
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Former opt-in payments					125,000
Janssen Biotech, Inc. | Second Opt-in Election
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Former opt-in payments					200,000
Janssen Biotech, Inc. | First Amendment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Increase (decrease) in transaction price					(9,400)
Janssen Biotech, Inc. | First Amendment | License and Collaborative Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized		25,000			5,300
Janssen Biotech, Inc. | First Amendment | Development Milestone Payments
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount to be received	50,000				50,000
Janssen Biotech, Inc. | First Amendment | Milestone payment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized		$ 25,000
Janssen Biotech, Inc. | First Amendment | PTG-200, Phase 3 CD Clinical Trial Primary Clinical Endpoint
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount to be received							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]
Amount to be received							$ 115,000
Janssen Biotech, Inc. | First Amendment | Second-Generation Phase 2b Milestone Payment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount to be received	50,000				50,000
Janssen Biotech, Inc. | Amended First Opt-in | Upfront cash payment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount to be received	50,000				50,000
Janssen Biotech, Inc. | Amended Second Opt-in | Upfront cash payment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount to be received	50,000				$ 50,000
Janssen Biotech, Inc. | Estimated variable consideration | License and Collaborative Revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue	$ 17,400