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Stock-based Compensation - Compensation Cost Recognized for Stock Options (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
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)	$ 127	$ 355

X

- Definition

Compensation cost recognized for the stock options and restricted stock awards.

+ References

No definition available.

+ Details

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Namespace Prefix:	pstv_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- 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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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration