License and Collaboration Agreement - Narrative (Details) - USD ($)		3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 27, 2021	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021	Jun. 30, 2022	Apr. 30, 2022	Mar. 31, 2020	May 07, 2019	Jul. 13, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognized revenue from deferred revenue contract liability		$ 0	$ 200,000	$ 900,000	$ 1,700,000
Janssen | Services performed for IL-23 receptor antagonist compound research costs and other services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement for services performed							$ 17,900,000
Janssen | License and Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized		0	2,300,000	26,600,000	9,900,000
Janssen | License and Collaboration Agreement | Three phase 1 studies of second-generation compounds
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Eligible amount received						$ 7,500,000
Janssen | License and Collaboration Agreement | 3rd patient in the first Phase 2 clinical trial for any second-generation compound for a second indication
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Eligible amount received								$ 25,000,000.0
Janssen | License and Collaboration Agreement | Phase 3 clinical trial for a second-generation compound for any indication
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upcoming potential development milestones		115,000,000.0		115,000,000.0
Janssen | Original Agreement | License and Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Duration of the parties' research collaboration agreement	2 years
Janssen | Original Agreement | License and Collaboration Agreement | Three phase 1 studies of second-generation compounds
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Expense obligations eliminated	$ 5,000,000.0
Percentage of development cost eliminated	50.00%
Janssen | Original Agreement | License and Collaboration Agreement | Phase 2a and 2b costs for PTG-200
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development cost payable		100,000,000.0		$ 100,000,000.0
Development cost incurred, percentage				20.00%
Janssen | Original Agreement | License and Collaboration Agreement | Phase 2 studies for second-generation products
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of development cost eliminated	20.00%
Milestone payment											$ 50,000,000.0
Janssen | Original Agreement | License and Collaboration Agreement | Maximum | Three phase 1 studies of second-generation compounds
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development cost payable											20,000,000.0
Janssen | Original Agreement | License and Collaboration Agreement | Maximum | Phase 2a and 2b costs for PTG-200
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development cost payable											20,000,000.0
Janssen | Original Agreement | License and Collaboration Agreement | Maximum | Phase 2 studies for second-generation products
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development cost payable		25,000,000.0		$ 25,000,000.0
Expense obligations eliminated	$ 25,000,000.0
Janssen | Original Agreement | Upfront cash payment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment											$ 50,000,000.0
Janssen | Restated Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment							112,500,000
Development cost payable							6,900,000
Reimbursement for services performed							8,200,000
Maximum percentages royalties on net product sales	10.00%
Cumulative catch-up adjustment increasing license and collaboration revenue			8,000,000.0		8,000,000.0	$ 8,000,000.0
Final transaction price							$ 131,700,000
Janssen | Restated Agreement | Second-generation Oral Interleukin ("IL")-23 Receptor Antagonist Development Compound
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Eligible amount received									$ 5,000,000.0
Janssen | Restated Agreement | Phase 2a study evaluating PTG-200 for the treatment of CD
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development cost incurred, percentage				50.00%
Janssen | Restated Agreement | License and Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Eligible amount received										$ 25,000,000.0
Janssen | Restated Agreement | License and Collaboration Agreement | Phase 1 studies of second-generation compounds
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of development cost eliminated				100.00%
Janssen | Restated Agreement | License and Collaboration Agreement | Phase 2 studies for second-generation products
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upcoming potential development milestones		25,000,000.0		$ 25,000,000.0
Milestone payment	$ 25,000,000.0
Janssen | Restated Agreement | License and Collaboration Agreement | 3rd patient in the first Phase 2 clinical trial for any second-generation compound for a second indication
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upcoming potential development milestones		10,000,000.0		$ 10,000,000.0
Janssen | Restated Agreement | License and Collaboration Agreement | Phase 2a study evaluating PTG-200 for the treatment of CD
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development cost incurred, percentage				20.00%
Janssen | Restated Agreement | License and Collaboration Agreement | Dosing of the 3rd patient in a Phase 3 clinical trial for a second-generation compound for any indication
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upcoming potential development milestones		50,000,000.0		$ 50,000,000.0
Janssen | Restated Agreement | License and Collaboration Agreement | Dosing Of Third Patient In Phase 3 Clinical Trial For Second Generation Compound For A Second Indication [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upcoming potential development milestones		15,000,000.0		15,000,000.0
Janssen | Restated Agreement | License and Collaboration Agreement | Maximum | Phase 2a study evaluating PTG-200 for the treatment of CD
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development cost payable		$ 20,000,000.0		$ 20,000,000.0
Janssen | Restated Agreement | Additional Services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized			$ 0		$ 800,000