Defined Contribution Plan - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Compensation And Retirement Disclosure [Abstract]
Payment made under defined contribution plan	$ 0.4	$ 0.2
Defined Contribution Plan, Plan Name [Extensible List]	plrx:FourZeroOneKPlansMember	plrx:FourZeroOneKPlansMember

Indicates name of defined contribution plan designed to provide retirement benefits. Includes, but is not limited to, legal name of plan.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

Name:	us-gaap_DefinedContributionPlanPlanNameExtensibleList
Namespace Prefix:	us-gaap_
Data Type:	srt-types:extensibleListItemType
Balance Type:	na
Period Type:	duration