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Stockholders' Equity - Additional Information (Detail) (USD $) In Thousands, except Share data, unless otherwise specified | 9 Months Ended | 12 Months Ended | 0 Months Ended | 9 Months Ended |
--- | --- | --- | --- | --- | ---
Sep. 30, 2014 | Dec. 31, 2013 | Jun. 24, 2014 | Sep. 30, 2013 | Dec. 31, 2012
Reverse Stock split Ratio |  | 6.28 us-gaap_StockholdersEquityNoteStockSplitConversionRatio1 |  |  |
Net proceeds from initial public offering | $ 102,672 us-gaap_ProceedsFromIssuanceInitialPublicOffering |  |  |  |
Redeemable convertible preferred stock converted into common stock shares |  | 15,045,125 us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion |  |  | 11,782,007 us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion
Preferred stock, shares authorized | 5,000,000 us-gaap_PreferredStockSharesAuthorized | 0 us-gaap_PreferredStockSharesAuthorized |  |  |
Preferred stock, par value | $ 0.001 us-gaap_PreferredStockParOrStatedValuePerShare | $ 0.001 us-gaap_PreferredStockParOrStatedValuePerShare |  |  |
Common stock, shares authorized | 115,000,000 us-gaap_CommonStockSharesAuthorized | 115,000,000 us-gaap_CommonStockSharesAuthorized |  |  | 100,000,000 us-gaap_CommonStockSharesAuthorized
Common stock, par value | $ 0.001 us-gaap_CommonStockParOrStatedValuePerShare | $ 0.001 us-gaap_CommonStockParOrStatedValuePerShare |  |  | $ 0.001 us-gaap_CommonStockParOrStatedValuePerShare
Common Stock [Member] |  |  |  |  |
Reverse stock split, description | Company effected a 1-for-6.28 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of redeemable convertible preferred stock. |  |  |  |
Reacquired and retired shares of restricted common stock |  | (6,635)us-gaap_StockRepurchasedAndRetiredDuringPeriodShares / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember |  |  |
Common Stock [Member] | IPO [Member] |  |  |  |  |
Reverse Stock split Ratio | 0.1592 us-gaap_StockholdersEquityNoteStockSplitConversionRatio1 / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember / us-gaap_StatementScenarioAxis = us-gaap_IPOMember |  |  |  |
Initial public offering, common stock issued, shares |  |  | 6,900,000 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember / us-gaap_StatementScenarioAxis = us-gaap_IPOMember |  |
Initial public offering, common stock issued per share |  |  | $ 16.00 us-gaap_SharesIssuedPricePerShare / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember / us-gaap_StatementScenarioAxis = us-gaap_IPOMember |  |
Net proceeds from initial public offering |  |  | 102,672 us-gaap_ProceedsFromIssuanceInitialPublicOffering / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember / us-gaap_StatementScenarioAxis = us-gaap_IPOMember |  |
Offering costs |  |  | $ 2,508 zfgn_InitialPublicOfferingCost / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember / us-gaap_StatementScenarioAxis = us-gaap_IPOMember |  |
Redeemable convertible preferred stock converted into common stock shares | 15,077,621 us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember / us-gaap_StatementScenarioAxis = us-gaap_IPOMember |  |  |  |
Common Stock [Member] | Restricted Stock [Member] |  |  |  |  |
Reacquired and retired shares of restricted common stock |  | 6,635 us-gaap_StockRepurchasedAndRetiredDuringPeriodShares / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember |  | 6,635 us-gaap_StockRepurchasedAndRetiredDuringPeriodShares / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember |

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- Definition Face amount or stated value per share of preferred stock nonredeemable or redeemable solely at the option of the issuer. + 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 210.5-02.28) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29 -Article 5 + Details
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