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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