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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) | 9 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Summary Of Significant Accounting Policies [Line Items] |  |  |
Short-term investments maturity, description | Ninety days or less | Ninety days or less |
Deferred offering costs | $ 0 us-gaap_DeferredOfferingCosts | $ 743,000 us-gaap_DeferredOfferingCosts | $ 0 us-gaap_DeferredOfferingCosts
Estimated useful life | 5 years | 5 years |
Impairment losses on long-lived assets | 0 us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill | 0 us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill |
Percentage of likelihood to be realized upon ultimate settlement, description | 50.00%zfgn_LikelihoodPercentageMinimum | 50.00%zfgn_LikelihoodPercentageMinimum |
Quoted prices in active markets, Level 1 [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Cash equivalents carried at fair value | $ 100,000,000 us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | $ 26,501,000 us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | $ 8,001,000 us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member

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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash -URI http://asc.fasb.org/extlink&oid=6506951 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash Equivalents -URI http://asc.fasb.org/extlink&oid=6507016 + Details
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- Definition Specific incremental costs directly attributable to a proposed or actual offering of securities which are deferred at the end of the reporting period. + 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 210.5-02.8) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 340 -SubTopic 10 -Section S99 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=27011391&loc=d3e105025-122735 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 5 -Section A + Details
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- Definition Amount of impairment loss resulting from write-down of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit to fair value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16373-109275 + Details
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- Definition Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment. + References No definition available. + Details
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- Definition The minimum likelihood percentage threshold used to determine the amount of tax benefit to be recognized. + References No definition available. + Details
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