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Summary of Significant Accounting Policies - Revenue Recognition (Details) | 9 Months Ended
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Sep. 30, 2021 item
Summary of Significant Accounting Policies |
Number of performance obligation | 1
Revenue, Practical Expedient [Abstract] |
Financing component | true

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- Definition The threshold number of performance obligation to allocate transaction price. + References No definition available. + Details
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Namespace Prefix: | ptgx_
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Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_RevenuePracticalExpedientAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Indicates (true false) whether practical expedient was elected not to adjust consideration for effect of financing component when transfer and customer payment for product or service occurs within one year or less. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 22 -URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130569-203045 + Details
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