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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   6.28us-gaap_StockholdersEquityNoteStockSplitConversionRatio1      
Net proceeds from initial public offering $ 102,672us-gaap_ProceedsFromIssuanceInitialPublicOffering        
Redeemable convertible preferred stock converted into common stock shares   15,045,125us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion     11,782,007us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion
Preferred stock, shares authorized 5,000,000us-gaap_PreferredStockSharesAuthorized 0us-gaap_PreferredStockSharesAuthorized      
Preferred stock, par value $ 0.001us-gaap_PreferredStockParOrStatedValuePerShare $ 0.001us-gaap_PreferredStockParOrStatedValuePerShare      
Common stock, shares authorized 115,000,000us-gaap_CommonStockSharesAuthorized 115,000,000us-gaap_CommonStockSharesAuthorized     100,000,000us-gaap_CommonStockSharesAuthorized
Common stock, par value $ 0.001us-gaap_CommonStockParOrStatedValuePerShare $ 0.001us-gaap_CommonStockParOrStatedValuePerShare     $ 0.001us-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 0.1592us-gaap_StockholdersEquityNoteStockSplitConversionRatio1
/ 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		    
Net proceeds from initial public offering     102,672us-gaap_ProceedsFromIssuanceInitialPublicOffering
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
/ us-gaap_StatementScenarioAxis
= us-gaap_IPOMember		    
Offering costs     $ 2,508zfgn_InitialPublicOfferingCost
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
/ us-gaap_StatementScenarioAxis
= us-gaap_IPOMember		    
Redeemable convertible preferred stock converted into common stock shares 15,077,621us-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   6,635us-gaap_StockRepurchasedAndRetiredDuringPeriodShares
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		   6,635us-gaap_StockRepurchasedAndRetiredDuringPeriodShares
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember