License agreement upfront payment.

No definition available.

Name:	phas_LicenseAgreementUpfrontPayment
Namespace Prefix:	phas_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

License Agreements.

+ References

No definition available.

+ Details

Name:	phas_LicenseAgreementsLineItems
Namespace Prefix:	phas_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Milestone Payment

+ References

No definition available.

+ Details

Name:	phas_MilestonePayment
Namespace Prefix:	phas_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Nonroyalty payment received.

+ References

No definition available.

+ Details

Name:	phas_NonroyaltyPaymentReceived
Namespace Prefix:	phas_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Potential milestones payment

+ References

No definition available.

+ Details

Name:	phas_PotentialMilestonesPayment
Namespace Prefix:	phas_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Purchase Obligation, to be Paid, Year Five Through Eleven

+ References

No definition available.

+ Details

Name:	phas_PurchaseObligationToBePaidYearFiveThroughEleven
Namespace Prefix:	phas_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Sale of licensed products or services.

+ References

No definition available.

+ Details

Name:	phas_SaleOfLicensedProductsOrServices
Namespace Prefix:	phas_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Sublicense Agreement, Upfront Payment

+ References

No definition available.

+ Details

Name:	phas_SublicenseAgreementUpfrontPayment
Namespace Prefix:	phas_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of cash outflow for fees classified as other.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3536-108585

+ Details

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

- Definition

The amount of cash paid for royalties during the current period.

+ References

+ Details

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

- Definition

Amount of purchase arrangement to be paid in fourth fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of purchase arrangement to be paid in second and third fiscal years following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

The aggregate costs incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 985
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128462-111756

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 730
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6420194&loc=d3e21568-108373

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=phas_AchievementOfClinicalDevelopmentAndRegulatoryMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=phas_CommercialMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=phas_AchievementOfCertainPreRevenueRegulatoryMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=phas_AchievementOfCertainCommercialMilestonesAndTieredRoyaltyPaymentsOnNetSalesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=phas_EMAApprovalOfApplicationForLicensedProductMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=phas_AchievementOfConditionalRegulatoryApprovalFromEMAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=phas_AchievementOfUnconditionalRegulatoryApprovalFromEMAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=phas_PaymentUponAchievementOfCertainDevelopmentAndIntellectualPropertyMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=phas_MedImmuneLimitedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=phas_AlfasigmaSpAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Licenses and Other Agreements - Additional Information (Details) - USD ($)	1 Months Ended			3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jan. 31, 2020	Nov. 30, 2017	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021
License Agreements [Line Items]
Research and development				$ 27,366,000	$ 20,856,000	$ 49,686,000	$ 32,305,000
Purchase obligation, year 2022 through 2023								$ 14,000,000.0
Purchase obligation, year 2024								37,000,000.0
Purchase obligation, year 2025 through 2031								$ 48,000,000.0
Duke University
License Agreements [Line Items]
Aggregate royalties payment						200,000
Sale of licensed products or services						0
Fees payment						300,000
Nonroyalty payment received						1,000,000.0
Research and development						0	0
Wacker
License Agreements [Line Items]
Research and development				100,000	100,000	200,000	200,000
Viamet Pharmaceuticals Holdings, LLC
License Agreements [Line Items]
Research and development				$ 0	$ 0	0	$ 100,000
License agreement upfront payment		$ 100,000
Achievement Of Clinical Development And Regulatory Milestones \| Duke University
License Agreements [Line Items]
Potential milestones payment						2,200,000
Commercial Milestones \| Duke University
License Agreements [Line Items]
Potential milestones payment						$ 400,000
Commercial Milestones \| Viamet Pharmaceuticals Holdings, LLC
License Agreements [Line Items]
Potential milestones payment		142,500,000
Achievement Of Certain Pre-Revenue Regulatory Milestones \| Alfasigma S.p.A.
License Agreements [Line Items]
Potential milestones payment	$ 35,000,000.0
Achievement Of Certain Commercial Milestones And Tiered Royalty Payments On Net Sales \| Alfasigma S.p.A.
License Agreements [Line Items]
Potential milestones payment	190,000,000.0
EMA Approval Of Application For Licensed Product \| Alfasigma S.p.A.
License Agreements [Line Items]
Potential milestones payment	10,000,000.0
Achievement Of Conditional Regulatory Approval From EMA \| Alfasigma S.p.A.
License Agreements [Line Items]
Potential milestones payment	12,500,000
Achievement Of Unconditional Regulatory Approval From EMA \| Alfasigma S.p.A.
License Agreements [Line Items]
Potential milestones payment	12,500,000
Achievement Of Certain Development And Intellectual Property Milestones \| Viamet Pharmaceuticals Holdings, LLC
License Agreements [Line Items]
Potential milestones payment		$ 5,100,000
MedImmune Limited
License Agreements [Line Items]
Milestone payment			$ 3,000,000.0
MedImmune Limited \| Achievement Of Clinical Development And Regulatory Milestones
License Agreements [Line Items]
Potential milestones payment			15,000,000.0
MedImmune Limited \| Commercial Milestones
License Agreements [Line Items]
Potential milestones payment			$ 50,000,000.0
Alfasigma S.p.A.
License Agreements [Line Items]
Upfront payment from sublicense agreement	$ 20,000,000.0