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Stock-based Compensation - Compensation Cost Recognized for Stock Options and Restricted Stock Awards (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
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Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2017
Compensation cost recognized for the stock options and restricted stock awards [Abstract] |  |  |  |
Total compensation cost for share-based payment arrangements recognized in the statement of operations (net of tax of $0) | $ 49 | $ 143 | $ 355 | $ 753

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- Definition Compensation cost recognized for the stock options and restricted stock awards. + References No definition available. + Details
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Namespace Prefix: | pstv_
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- Definition Amount of noncash expense for share-based payment arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585 + Details
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