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CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Parenthetical) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
Statement Of Stockholders Equity [Abstract] |  |
Sale of stock for common stock and pre funded warrants, offering costs |  | $ 0.6
Offering costs,net | $ 0.6 |

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

X
- Definition Pre funded warrants and stock issued during period value offering costs. + References No definition available. + Details
Name: | pstv_PreFundedWarrantsAndStockIssuedDuringPeriodValueOfferingCosts
Namespace Prefix: | pstv_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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