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CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Parenthetical) $ in Millions | 12 Months Ended
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Dec. 31, 2019 USD ($)
Statement Of Stockholders Equity [Abstract] |
Sale of stock for common stock and pre funded warrants, offering costs | $ 0.6

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- Definition Pre funded warrants and stock issued during period value offering 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_StatementOfStockholdersEquityAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration