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

Amount of required minimum rental payments for leases having an initial or remaining non-cancelable letter-terms in excess of one year.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 20

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 10

-Section 55

-Paragraph 40

-Subparagraph (Note 3)

-URI http://asc.fasb.org/extlink&oid=6584154&loc=d3e38371-112697

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41499-112717

+ Details

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

- Details

Name:	zfgn_CommitmentAndContingenciesLineItems
Namespace Prefix:	zfgn_
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

Operating Lease Option For Extension Period

+ References

No definition available.

+ Details

Name:	zfgn_OperatingLeaseOptionForExtensionPeriod
Namespace Prefix:	zfgn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=zfgn_OfficeAndLaboratorySpaceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TitleOfIndividualAxis=zfgn_OfficeSpaceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TitleOfIndividualAxis=zfgn_OfficeTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Commitments and Contingencies - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended		12 Months Ended			0 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	May 15, 2014
Commitment And Contingencies [Line Items]
Future minimum commitments	$ 660		$ 64
Operating lease, rental expense	104us-gaap_OperatingLeasesRentExpenseNet	87us-gaap_OperatingLeasesRentExpenseNet	105us-gaap_OperatingLeasesRentExpenseNet	129us-gaap_OperatingLeasesRentExpenseNet	102us-gaap_OperatingLeasesRentExpenseNet
Laboratory Space [Member]
Commitment And Contingencies [Line Items]
Future minimum commitments			0
Development Based Milestones [Member]
Commitment And Contingencies [Line Items]
Milestone payments due	6,700
Pre Commercialization Milestones [Member]
Commitment And Contingencies [Line Items]
Milestone payments	12,250		18,950
Product Commercialization Milestones [Member]
Commitment And Contingencies [Line Items]
Milestone payments	12,500		12,500
Subsequent Licensed Product [Member]
Commitment And Contingencies [Line Items]
Licensing fees per product maximum	$ 1,250		$ 1,250
Office Two [Member]
Commitment And Contingencies [Line Items]
Lease expiration date	Jul. 31, 2017
Lease extension period						3 years
Office Space [Member]
Commitment And Contingencies [Line Items]
Lease expiration date			Jul. 31, 2014
Lease extension period			6 months
Office Space [Member] \| Before Amendment [Member]
Commitment And Contingencies [Line Items]
Lease expiration date			Jul. 31, 2013
Office Space [Member] \| After Amendment [Member]
Commitment And Contingencies [Line Items]
Lease expiration date			Jan. 31, 2014