Fair value adjustment of tranche liability.

No definition available.

Name:	ptgx_FairValueAdjustmentOfTrancheLiability
Namespace Prefix:	ptgx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Probability of exercisability percentage.

+ References

No definition available.

+ Details

Name:	ptgx_ProbabilityOfExercisabilityPercentage
Namespace Prefix:	ptgx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Redeemable convertible preferred stock liability fair value.

+ References

No definition available.

+ Details

Name:	ptgx_RedeemableConvertiblePreferredStockLiabilityFairValue
Namespace Prefix:	ptgx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Redeemable Convertible Preferred Stock Tranche Liability [Line Items]

+ References

No definition available.

+ Details

Name:	ptgx_RedeemableConvertiblePreferredStockTrancheLiabilityLineItems
Namespace Prefix:	ptgx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Settlement of fair value of redeemable convertible preferred stock liability.

+ References

No definition available.

+ Details

Name:	ptgx_SettlementOfFairValueOfRedeemableConvertiblePreferredStockLiability
Namespace Prefix:	ptgx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Temporary equity shares issue price.

+ References

No definition available.

+ Details

Name:	ptgx_TemporaryEquitySharesIssuePrice
Namespace Prefix:	ptgx_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Aggregate number of nonredeemable convertible preferred shares reserved for future issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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_ConvertiblePreferredStockSharesReservedForFutureIssuance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Period the instrument, asset or liability is expected to be outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueAssumptionsExpectedTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Measure of dispersion, in percentage terms (for instance, the standard deviation or variance), for a given stock price.

+ References

+ Details

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

- Definition

Risk-free interest rate assumption used in valuing an instrument.

+ References

+ Details

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

- Definition

Weighted average expected volatility of stock price.

+ References

+ Details

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

- Definition

The cash inflow from issuance of callable preferred stock which is identified as being convertible to another type of financial security at the option of the issuer or the holder.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3255-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Financing Activities
-URI http://asc.fasb.org/extlink&oid=6513228

+ Details

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

- Definition

The maximum number of securities classified as temporary equity that are permitted to be issued by an entity's charter and bylaws. Temporary equity is a security with redemption features that are outside the control of the issuer, is not classified as an asset or liability in conformity with GAAP, and is not mandatorily redeemable. Includes any type of security that is redeemable at a fixed or determinable price or on a fixed or determinable date or dates, is redeemable at the option of the holder, or has conditions for redemption which are not solely within the control of the issuer. If convertible, the issuer does not control the actions or events necessary to issue the maximum number of shares that could be required to be delivered under the conversion option if the holder exercises the option to convert the stock to another class of equity. If the security is a warrant or a rights issue, the warrant or rights issue is considered to be temporary equity if the issuer cannot demonstrate that it would be able to deliver upon the exercise of the option by the holder in all cases. Includes stock with put option held by ESOP and stock redeemable by holder only in the event of a change in control of the issuer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(27)(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

The number of securities classified as temporary equity that have been sold (or granted) to the entity's shareholders. Securities issued include securities outstanding and securities held in treasury. Temporary equity is a security with redemption features that are outside the control of the issuer, is not classified as an asset or liability in conformity with GAAP, and is not mandatorily redeemable. Includes any type of security that is redeemable at a fixed or determinable price or on a fixed or determinable date or dates, is redeemable at the option of the holder, or has conditions for redemption which are not solely within the control of the issuer. If convertible, the issuer does not control the actions or events necessary to issue the maximum number of shares that could be required to be delivered under the conversion option if the holder exercises the option to convert the stock to another class of equity. If the security is a warrant or a rights issue, the warrant or rights issue is considered to be temporary equity if the issuer cannot demonstrate that it would be able to deliver upon the exercise of the option by the holder in all cases. Includes stock with put option held by ESOP and stock redeemable by holder only in the event of a change in control of the issuer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(27)(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_ValuationTechniqueAxis=ptgx_OptionPricingModelMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=ptgx_ProbabilityWeightedExpectedReturnMethodMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Redeemable Convertible Preferred Stock Tranche Liability - Additional Information (Detail) - USD ($)		1 Months Ended			12 Months Ended
	Jul. 10, 2015	Apr. 30, 2016	Mar. 31, 2016	Aug. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jul. 31, 2015
Redeemable Convertible Preferred Stock Tranche Liability [Line Items]
Redeemable convertible preferred stock, shares issued					0	77,185,117
Redeemable convertible preferred stock, proceeds					$ 22,488,000	$ 17,362,000	$ 9,000,000
Fair value of redeemable convertible preferred stock liability reclassified			$ 5,800,000	$ 2,300,000	$ 5,837,000		2,308,000
Redeemable convertible preferred stock, authorized					0	126,374,911
Change in fair value of redeemable convertible preferred stock tranche liabilities					$ 4,194,000	$ 626,000	897,000
Second Tranche Warrants [Member]
Redeemable Convertible Preferred Stock Tranche Liability [Line Items]
Probability of exercisability percentage						0.00%
Series C Redeemable Convertible Preferred Stock [Member]
Redeemable Convertible Preferred Stock Tranche Liability [Line Items]
Redeemable convertible preferred stock, shares issued	35,147,617		45,189,794
Redeemable convertible preferred stock, proceeds	$ 17,500,000		$ 22,500,000
Fair value of redeemable convertible preferred stock liability	$ 1,000,000		$ 5,800,000			$ 1,600,000
Probability of exercisability percentage	90.00%					95.00%
Fair value assumptions, expected term	1 year		2 years			6 months 11 days
Fair value assumptions, risk-free interest rate	0.50%		0.73%			0.90%
Redeemable convertible preferred stock, price per share						$ 0.4979		$ 0.4979
Redeemable convertible preferred stock, authorized						80,337,411		80,337,411
Fair value assumptions, expected volatility			52.00%
Change in fair value of redeemable convertible preferred stock tranche liabilities					$ 4,200,000	$ 626,000	$ 897,000
Series C Redeemable Convertible Preferred Stock [Member] \| Option Pricing Model [Member]
Redeemable Convertible Preferred Stock Tranche Liability [Line Items]
Fair value assumptions, weighted volatility rate			67.00%
Series C Redeemable Convertible Preferred Stock [Member] \| PWERM [Member]
Redeemable Convertible Preferred Stock Tranche Liability [Line Items]
Fair value assumptions, weighted volatility rate			33.00%
Series C Redeemable Convertible Preferred Stock [Member] \| Second Tranche Warrants [Member]
Redeemable Convertible Preferred Stock Tranche Liability [Line Items]
Additional shares to be issued upon the achievement of certain development milestones	45,189,794
Series B Redeemable Convertible Preferred Stock [Member]
Redeemable Convertible Preferred Stock Tranche Liability [Line Items]
Redeemable convertible preferred stock, shares issued		1,999,998		18,000,000
Redeemable convertible preferred stock, proceeds		$ 20,000		$ 9,000,000
Fair value of redeemable convertible preferred stock liability				$ 2,300,000
Probability of exercisability percentage				100.00%
Fair value assumptions, expected term				0 years
Fair value assumptions, risk-free interest rate				0.50%
Redeemable convertible preferred stock, price per share				$ 0.50		$ 0.5000
Redeemable convertible preferred stock, authorized						40,000,000