Charge on non-transferable subscription rights.

No definition available.

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

- Definition

Conversion price per share.

+ References

No definition available.

+ Details

Name:	pstv_ConversionPricePerShare
Namespace Prefix:	pstv_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Gross proceeds from issuance of private placement.

+ References

No definition available.

+ Details

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

- Definition

Number Of Preferred Stock Shares Exercised.

+ References

No definition available.

+ Details

Name:	pstv_NumberOfPreferredStockSharesExercised
Namespace Prefix:	pstv_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of warrants exercised

+ References

No definition available.

+ Details

Name:	pstv_NumberOfWarrantsExercised
Namespace Prefix:	pstv_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Preferred Stock [Abstract]

+ References

No definition available.

+ Details

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

- Definition

Exercise price per share or per unit of warrants or rights outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(i)(4))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of securities into which the class of warrant or right may be converted. For example, but not limited to, 500,000 warrants may be converted into 1,000,000 shares.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(i)(2))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of warrants or rights outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_ClassOfWarrantOrRightOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Carrying value as of the balance sheet date of dividends declared but unpaid on equity securities issued by the entity and outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(24))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(15)(5))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(20))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(15)(5))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

+ Details

Name:	us-gaap_DividendsPayableCurrentAndNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Per share decrease in conversion price of convertible preferred stock. Excludes change due to standard antidilution provision.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21475-112644

+ Details

Name:	us-gaap_PreferredStockConvertibleConversionPriceDecrease
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage rate used to calculate dividend payments on preferred stock.

+ References

No definition available.

+ Details

Name:	us-gaap_PreferredStockDividendRatePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Face amount or stated value per share of preferred stock nonredeemable or redeemable solely at the option of the issuer.

+ References

+ Details

Name:	us-gaap_PreferredStockParOrStatedValuePerShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

The maximum number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) permitted to be issued by an entity's charter and bylaws.

+ References

+ Details

Name:	us-gaap_PreferredStockSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Aggregate share number for all nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) held by stockholders. Does not include preferred shares that have been repurchased.

+ References

+ Details

Name:	us-gaap_PreferredStockSharesOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of new stock issued during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_StockIssuedDuringPeriodSharesNewIssues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Per share decrease in exercise price of warrant. Excludes change due to standard antidilution provision.

+ References

+ Details

Name:	us-gaap_WarrantExercisePriceDecrease
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_ClassOfWarrantOrRightAxis=pstv_TwoThousandEighteenRightsOfferingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_WarrantMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=pstv_SeriesAConvertiblePreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=pstv_SeriesBConvertiblePreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=pstv_SeriesCConvertiblePreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DividendsAxis=us-gaap_DividendPaidMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=pstv_CommonStockAndSeriesBConvertiblePreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=pstv_SeriesTWarrantsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=pstv_FiftySeriesTWarrantsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=pstv_TwoThousandEighteenRightsOfferingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ClassOfWarrantOrRightAxis=pstv_SeriesTWarrantsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ClassOfWarrantOrRightAxis=pstv_SeriesCConvertiblePreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Stockholders' Equity - Preferred Stock (Details) - USD ($)					12 Months Ended
	Sep. 25, 2019	Aug. 02, 2019	Jul. 25, 2018	Jul. 17, 2018	Dec. 31, 2019	Dec. 31, 2018	Aug. 06, 2019	Aug. 05, 2019	May 24, 2018	May 23, 2018
Preferred Stock [Abstract]
Preferred stock, shares authorized (in shares)					5,000,000	5,000,000	5,000,000	5,000,000	5,000,000	5,000,000
Preferred stock, par value (in dollars per share)					$ 0.001	$ 0.001
Preferred stock, shares outstanding (in shares)					1,959	4,606
Number of shares callable by warrants (in shares)									2,300,000	23,400,000
2018 Rights Offering [Member]
Preferred Stock [Abstract]
Number of shares callable by warrants (in shares)	1			1
Common stock issued (in shares)			6,723
Gross proceeds from private placement of stock			$ 5,700,000
2018 Rights Offering [Member] \| Outstanding Warrants [Member]
Preferred Stock [Abstract]
Number of shares callable by warrants (in shares)				1,050
Warrant exercise price (in dollars per share)				$ 1,000
Series A Convertible Preferred Stock [Member]
Preferred Stock [Abstract]
Preferred stock, shares outstanding (in shares)					0	0
Convertible preferred stock					3.60%	3.60%
Series B Convertible Preferred Stock [Member]
Preferred Stock [Abstract]
Preferred stock, shares outstanding (in shares)					1,021	1,112
Series C Convertible Preferred Stock [Member]
Preferred Stock [Abstract]
Preferred stock, shares authorized (in shares)			7,000
Preferred stock, par value (in dollars per share)			$ 0.001
Preferred stock, shares outstanding (in shares)					938	3,494
Common stock issued (in shares)			6,723
Preferred stock conversion price (in dollars per share)					$ 39.93
Preferred stock conversion price reduced (in dollars per share)					$ 7.50
Series C Convertible Preferred Stock [Member] \| Dividend Paid [Member]
Preferred Stock [Abstract]
Dividends payable		$ 554,000			$ 554,000	$ 2,500,000
Series C Convertible Preferred Stock [Member] \| 2018 Rights Offering [Member]
Preferred Stock [Abstract]
Number of shares callable by warrants (in shares)				20,000
Preferred stock conversion price (in dollars per share)	$ 7.50	$ 39.93
Preferred stock conversion price reduced (in dollars per share)	3.2132	$ 7.50
Common Stock and Series B Convertible Preferred Stock [Member] \| 2018 Rights Offering [Member]
Preferred Stock [Abstract]
Charge on Non Transferable Subscription Rights				$ 0
Series T Warrants [Member] \| 2018 Rights Offering [Member]
Preferred Stock [Abstract]
Number of shares callable by warrants (in shares)		1
Warrant exercise price (in dollars per share)		$ 0.7986
Warrant exercise price reduced (in dollars per share)		$ 0.15
Series T Warrants [Member] \| 2018 Rights Offering [Member] \| Outstanding Warrants [Member]
Preferred Stock [Abstract]
Number of shares callable by warrants (in shares)		50
Warrant exercise price (in dollars per share)	$ 3.2132
50 Series T Warrants [Member] \| 2018 Rights Offering [Member]
Preferred Stock [Abstract]
Warrant exercise price (in dollars per share)		$ 7.50
2018 Rights Offering [Member] \| Series T Warrants [Member] \| Outstanding Warrants [Member]
Preferred Stock [Abstract]
Class of warrant outstanding					3,788,400
Number of warrants exercised					75,768
2018 Rights Offering [Member] \| Series C Convertible Preferred Stock [Member]
Preferred Stock [Abstract]
Preferred stock, shares outstanding (in shares)					938
Number of preferred stock, shares exercised					291,920