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Revenue Recognition - Additional Information (Details) $ in Millions | 3 Months Ended
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Mar. 31, 2020 USD ($)
Revenue Recognition [Abstract] |
Development revenue and related costs | $ 0.1

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- Definition Development revenue and related costs. + References No definition available. + Details
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Namespace Prefix: | pstv_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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