Name:	prta_AllocatedConsiderationtoPerformanceObligations
Namespace Prefix:	prta_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Collaboration Agreement, Allocation of Initial Transaction Price

No definition available.

Name:	prta_CollaborationAgreementAllocationOfInitialTransactionPrice
Namespace Prefix:	prta_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Collaboration Agreement, Exercise Fee (US Rights) per Program

No definition available.

Name:	prta_CollaborationAgreementExerciseFeeUSRightsPerProgram
Namespace Prefix:	prta_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Collaboration Agreement, Option Fees and Milestone Payments, Payment Term

No definition available.

Name:	prta_CollaborationAgreementOptionFeesandMilestonePaymentsPaymentTerm
Namespace Prefix:	prta_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Collaboration Agreement, Potential Commercial Milestone Payments Per Program

No definition available.

Name:	prta_CollaborationAgreementPotentialCommercialMilestonePaymentsPerProgram
Namespace Prefix:	prta_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Collaboration Agreement, Potential Development and Regulatory Milestone Payments

No definition available.

Name:	prta_CollaborationAgreementPotentialDevelopmentAndRegulatoryMilestonePayments
Namespace Prefix:	prta_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Collaboration Agreement, Potential Regulatory Milestone Payments Per Program

No definition available.

Name:	prta_CollaborationAgreementPotentialRegulatoryMilestonePaymentsPerProgram
Namespace Prefix:	prta_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Collaboration Agreement, Potential Regulatory and Commercial Milestone Payments Per Program

No definition available.

Name:	prta_CollaborationAgreementPotentialRegulatoryandCommercialMilestonePaymentsPerProgram
Namespace Prefix:	prta_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Collaboration Agreement, Total Transaction Price, PRX019 US

No definition available.

Name:	prta_CollaborationAgreementTotalTransactionPricePRX019US
Namespace Prefix:	prta_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Collaboration Agreement, Total Transaction Price, tau/BMS-986446 Global

No definition available.

Name:	prta_CollaborationAgreementTotalTransactionPriceTauBMS986446Global
Namespace Prefix:	prta_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Collaboration Agreement, Upfront Payment

No definition available.

Name:	prta_CollaborationAgreementUpfrontPayment
Namespace Prefix:	prta_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Collaboration Program, Discount Rate

No definition available.

Name:	prta_CollaborationProgramDiscountRate
Namespace Prefix:	prta_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

Collaboration Program, Option Exercise Fee

No definition available.

Name:	prta_CollaborationProgramOptionExerciseFee
Namespace Prefix:	prta_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Collaboration Program, US License Agreement, Upfront Payment

No definition available.

Name:	prta_CollaborationProgramUSLicenseAgreementUpfrontPayment
Namespace Prefix:	prta_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Amount, after allowance for credit loss, of right to consideration from customer for product sold and service rendered in normal course of business, classified as current.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481990/310-10-45-2

Name:	us-gaap_AccountsReceivableNetCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 808
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479402/808-10-50-1

Name:	us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Amount of deferred income and obligation to transfer product and service to customer for which consideration has been received or is receivable.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(26)(c))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

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

Amount of deferred income and obligation to transfer product and service to customer for which consideration has been received or is receivable, classified as current.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(20))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

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

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 40
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-40

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-42

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 41
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-41

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (ee)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-32

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 30
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-30

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 22
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-22

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 22
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-22

Reference 8: http://www.xbrl.org/2003/role/exampleRef
-Topic 220
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147476153/220-40-55-4

Reference 9: http://www.xbrl.org/2003/role/exampleRef
-Topic 220
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 14
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147476153/220-40-55-14

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-7

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 270
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482964/270-10-50-1

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 924
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 11.L)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479941/924-10-S99-1

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-5

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-4

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

Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (a)
-SubTopic 10
-Topic 606
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-13

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

Cash received on stock transaction after deduction of issuance costs.

No definition available.

Name:	us-gaap_SaleOfStockConsiderationReceivedOnTransaction
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The number of shares issued or sold by the subsidiary or equity method investee per stock transaction.

No definition available.

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

Per share amount received by subsidiary or equity investee for each share of common stock issued or sold in the stock transaction.

No definition available.

Name:	us-gaap_SaleOfStockPricePerShare
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	instant

Name:	srt_ProductOrServiceAxis=prta_CollaborationProgramGlobalRightsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_ProductOrServiceAxis=prta_CollaborationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_CounterpartyNameAxis=prta_BristolMyersSquibbMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_ProductOrServiceAxis=prta_CollaborationProgramUSRightsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_ProductOrServiceAxis=prta_CollaborationProgramUSTauBMS986446Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_ProductOrServiceAxis=prta_CollaborationProgramGlobalTauBMS986446Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_ProductOrServiceAxis=prta_CollaborationProgramGlobalPRX019Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_CounterpartyNameAxis=prta_BMSFormerlyCelgeneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_SubsidiarySaleOfStockAxis=us-gaap_PrivatePlacementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: