Novartis Collaboration and License Agreement ("the Novartis Agreement") (Details)	1 Months Ended	3 Months Ended			6 Months Ended		12 Months Ended
	Oct. 31, 2019 class	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($) research
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue — related party		$ 1,789,000	$ 3,600,000		$ 3,963,000	$ 32,538,000
Novartis | Novartis Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of Integrin research targets (up to) | class	3
Proceeds from upfront non-refundable license fee							$ 50,000,000.0
Novartis | Novartis Agreement | Achievement of First Patient Dosing Milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue recognized from related party		$ 0		$ 25,000,000.0	25,000,000.0
Novartis | Novartis Agreement, R&D For PLN-1474 | Research and development expenses
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Transaction price of Novartis agreement (up to)							19,600,000
Novartis | Novartis Agreement, R&D For Integrin Research Targets | Research and development expenses
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Transaction price of Novartis agreement (up to)							$ 13,400,000
Maximum | Novartis | Novartis Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of Integrin research targets (up to) | research							3
Collaboration and license agreement, integrin research targets amount (up to)							$ 416,000,000.0
Amount remaining for achievement					$ 391,000,000.0