Significant Agreements - Novo Nordisk Share Purchase Agreement (Details) - Novo Nordisk - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2025	Jun. 30, 2026	Dec. 31, 2025	Jul. 08, 2021
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Share Purchase Agreement, Upfront Payment				$ 60,000,000.0
Share Purchase Agreement, Potential Payment Upon Achievement of Development and Sales Milestones		$ 1,080,000,000.00
Revenue from License and Intellectual Property, Share Purchase Agreement	$ 0	(50,000,000.0)
Milestone achievement, Clinical milestone		50,000,000.0
Accounts Receivable, after Allowance for Credit Loss		$ 0	$ 0

Milestone Achievement, Clinical Milestone -recognized where milestone was achieved at the end of the period presented.

No definition available.

Name:	prta_MilestoneAchievementClinicalMilestone
Namespace Prefix:	prta_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Revenue from License and Intellectual Property, Share Purchase Agreement

+ References

No definition available.

+ Details

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

- Definition

Share Purchase Agreement, Potential Payment Upon Achievement of Development and Sales Milestones

+ References

No definition available.

+ Details

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

- Definition

Share Purchase Agreement, Upfront Payment

+ References

No definition available.

+ Details

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

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 946
-SubTopic 310
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477802/946-310-45-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(5))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-04(5)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479170/946-210-S99-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 954
-SubTopic 310
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479196/954-310-45-1

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(3))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

+ Details

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

- Definition

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.

+ References

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

+ Details

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

- Details

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