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Nature of the Business and Basis of Presentation - Additional Information (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	9 Months Ended	12 Months Ended	0 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Jun. 24, 2014	Dec. 31, 2012
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Entity incorporation date	Nov. 22, 2005	Nov. 22, 2005
Accumulated deficit	$ (94,454)us-gaap_RetainedEarningsAccumulatedDeficit	$ (68,907)us-gaap_RetainedEarningsAccumulatedDeficit		$ (54,880)us-gaap_RetainedEarningsAccumulatedDeficit
Net proceeds from initial public offering	102,672us-gaap_ProceedsFromIssuanceInitialPublicOffering
Maximum [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Net proceeds from initial public offering		35,000us-gaap_ProceedsFromIssuanceInitialPublicOffering / us-gaap_RangeAxis = us-gaap_MaximumMember
Common Stock [Member] | IPO [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Net proceeds from initial public offering			102,672us-gaap_ProceedsFromIssuanceInitialPublicOffering / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember / us-gaap_StatementScenarioAxis = us-gaap_IPOMember
Initial public offering, common stock issued, shares			6,900,000us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember / us-gaap_StatementScenarioAxis = us-gaap_IPOMember
Initial public offering, common stock issued per share			$ 16.00us-gaap_SharesIssuedPricePerShare / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember / us-gaap_StatementScenarioAxis = us-gaap_IPOMember
Offering costs incurred			$ 2,508zfgn_InitialPublicOfferingCost / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember / us-gaap_StatementScenarioAxis = us-gaap_IPOMember

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+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228



Reference 2: 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=31042434&loc=d3e3255-108585

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 31

-Article 5



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

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-Name Regulation S-X (SX)

-Number 210

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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-Topic 210

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-Section S99

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-Number 210

-Section 04

-Article 3



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

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-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644



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