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CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Parenthetical) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2021 | Dec. 31, 2020
Statement Of Stockholders Equity [Abstract] |  |
Offering costs,net | $ 0.3 | $ 0.6

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- Definition Amount of Issuance of net of offering cost. + References No definition available. + Details
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Namespace Prefix: | pstv_
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Balance Type: | debit
Period Type: | duration

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