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CONDENSED STATEMENTS OF CASH FLOWS (UNAUDITED) (Parenthetical) - USD ($) $ in Millions | 9 Months Ended
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Sep. 30, 2023 | Sep. 30, 2022
Statement of Cash Flows [Abstract] |  |
Offering cost | $ 0.2 | $ 0.7

X
- Definition Offering costs. + References No definition available. + Details
Name: | pstv_OfferingCosts
Namespace Prefix: | pstv_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_StatementOfCashFlowsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration