Date which lease or group of leases is set to expire, in CCYY-MM-DD format.

No definition available.

Name:	us-gaap_LeaseExpirationDate1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Term of lessee's operating lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41499-112717

+ Details

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

- Definition

The total amount of sublease rental income recognized during the period that reduces the entity's rent expense incurred under operating leases.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 10
-Section 55
-Paragraph 40
-Subparagraph (Note 3)
-URI http://asc.fasb.org/extlink&oid=82846649&loc=d3e38371-112697

+ Details

Name:	us-gaap_OperatingLeasesRentExpenseSubleaseRentals1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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

No definition available.

+ Details

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

- Definition

Maximum fee the Company is obligated to pay per licensed product.

+ References

No definition available.

+ Details

Name:	zfgn_LicensingFeesPerProductMaximum
Namespace Prefix:	zfgn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of payment due for milestones that have been achieved

+ References

No definition available.

+ Details

Name:	zfgn_MilestoneLiability
Namespace Prefix:	zfgn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of payment due upon reaching milestone.

+ References

No definition available.

+ Details

Name:	zfgn_MilestonePaymentAmount
Namespace Prefix:	zfgn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The number of options available to extend an operating lease.

+ References

No definition available.

+ Details

Name:	zfgn_OperatingLeasesNumberOfOptionsToExtendLease
Namespace Prefix:	zfgn_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Sublease area of real estate property.

+ References

No definition available.

+ Details

Name:	zfgn_SubleaseAreaOfRealEstateProperty
Namespace Prefix:	zfgn_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	instant

- Definition

Sublease Expiration Date

+ References

No definition available.

+ Details

Name:	zfgn_SubleaseExpirationDate
Namespace Prefix:	zfgn_
Data Type:	xbrli:normalizedStringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=zfgn_SubsequentLicensedProductMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=zfgn_DevelopmentBasedMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LeaseArrangementTypeAxis=zfgn_OfficeSpaceLeaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_LeaseArrangementTypeAxis=zfgn_AdditionalOfficeSpaceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LeaseArrangementTypeAxis=zfgn_AdditionalOfficeSpaceTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=zfgn_PreCommercializationMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
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Commitments and Contingencies - Additional Information (Detail)					3 Months Ended		6 Months Ended
	Jan. 01, 2017 ft²	Oct. 01, 2015	Apr. 15, 2015 Option	Jul. 28, 2014	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)
Other Commitments [Line Items]
Operating lease, rental expense					$ 100,000	$ 100,000	$ 200,000	$ 200,000
Subsequent Licensed Product [Member]
Other Commitments [Line Items]
Licensing fees per product maximum					1,300,000		1,300,000
Development Based Milestones [Member]
Other Commitments [Line Items]
Milestone payments due					0	$ 0	$ 0	$ 0
Boston Office Space [Member]
Other Commitments [Line Items]
Lease expiration date				Jul. 31, 2020			Jul. 31, 2020
Boston Office Space [Member] \| Unrelated Third Party Member
Other Commitments [Line Items]
Subleased area \| ft²	2,976
Sublease expiration date	Jun. 30, 2019
Sublease rental income							$ 300,000
Additional Boston Office Space [Member]
Other Commitments [Line Items]
Lease expiration date			Jul. 31, 2020				Jul. 31, 2020
Lease extension period			3 years
Number of options to extend lease \| Option			1
San Diego Office Space [Member]
Other Commitments [Line Items]
Lease expiration date		Sep. 30, 2019
Lease extension period		5 years
Maximum [Member] \| Pre Commercialization Milestones [Member]
Other Commitments [Line Items]
Milestone payments					12,300,000		$ 12,300,000
Maximum [Member] \| Product Commercialization Milestones [Member]
Other Commitments [Line Items]
Milestone payments					$ 12,500,000		$ 12,500,000