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NanoTx License Agreement - Additional Information (Details) - NanoTx License Agreement [Member]	Mar. 29, 2020 USD ($)
License fee payment in cash	$ 400,000
License fee payment in common stock	300,000
Development and Sales Milestone [Member]
Milestone payments	$ 136,500,000

X

- Definition

Milestone Payments.

+ References

No definition available.

+ Details

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Namespace Prefix:	pstv_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Stock issued during period, value, license.

+ References

No definition available.

+ Details

Name:	pstv_StockIssuedDuringPeriodValueLicense
Namespace Prefix:	pstv_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

The amount of cash paid for royalties during the current period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3536-108585

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=pstv_NanoTxLicensesAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_IncomeStatementLocationAxis=pstv_DevelopmentAndSalesMilestoneMember
Namespace Prefix:
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Balance Type:
Period Type: