Defined Contribution Plan - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Compensation And Retirement Disclosure [Abstract]
Payment made under defined contribution plan	$ 0.1	$ 0.1	$ 0.3	$ 0.2
Defined Contribution Plan, Plan Name [Extensible List]	plrx:FourZeroOneKPlanMember	plrx:FourZeroOneKPlanMember	plrx:FourZeroOneKPlanMember	plrx:FourZeroOneKPlanMember