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Fair Value Measurements - Additional Information (Details) | 6 Months Ended
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Jun. 30, 2021
Series U Warrants [Member] |
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items] |
Warrants issued, underwritten public offering, period | 2019-09

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- Definition Warrants issued, underwritten public offering, period. + References No definition available. + Details
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Namespace Prefix: | pstv_
Data Type: | xbrli:gYearMonthItemType
Balance Type: | na
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_StatementEquityComponentsAxis=pstv_SeriesUWarrantsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |