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CONSOLIDATED CONDENSED STATEMENTS OF STOCKHOLDERS' EQUITY (UNAUDITED) (Parenthetical) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($)
Statement Of Stockholders Equity [Abstract]
Offering cost, net	$ 0.1

X

- Definition

Amount of sale of net of offering cost.

+ References

No definition available.

+ Details

Name:	pstv_SaleOfNetOfOfferingCost
Namespace Prefix:	pstv_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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