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Stock-Based Awards - Additional Information (Detail) (USD $) In Thousands, except Share data, unless otherwise specified | 0 Months Ended | 12 Months Ended | 9 Months Ended | 0 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Sep. 30, 2014 | Sep. 01, 2014 | Dec. 31, 2010 | Jun. 05, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Aggregate intrinsic value of stock options exercised |  | $ 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue | $ 12 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue | $ 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue |  |  |  |
Proceeds from exercise of stock options |  |  | 13 us-gaap_ProceedsFromStockOptionsExercised |  |  |  |  |
Weighted average grant-date fair value of stock options granted |  | $ 1.83 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue | $ 0.00 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue | $ 1.07 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue |  |  |  |
Unrecognized stock-based compensation cost | 6,224 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized | 1,334 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized |  |  | 6,224 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized |  |  |
Stock-based compensation recognized weighted average period | 3 years 4 months 24 days | 3 years 7 months 6 days |  |  |  |  |  |
Shares of common stock reserved for issuance |  | 16,726,699 us-gaap_CommonStockCapitalSharesReservedForFutureIssuance |  |  |  |  |  |
Common Stock [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Stock options granted |  |  |  |  | 529,565 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember |  |  |
Service Based Awards [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Outstanding unvested service-based stock options held by nonemployees |  | 17,513 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = zfgn_ServiceBasedAwardsMember |  |  |  |  |  |
Performance Shares [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Outstanding unvested service-based stock options held by nonemployees |  | 796 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |
Restricted Stock [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Outstanding unvested service-based stock options held by nonemployees |  |  | 10,615 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember | 24,548 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |  |  | 38,481 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |
Aggregate intrinsic value of restricted stock awards vested during the period |  | $ 38 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember | $ 22 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember | $ 22 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |  |  |  |
Nonemployee Service Based Options [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Number of outstanding unvested shares held by nonemployees | 17,513 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares / us-gaap_AwardTypeAxis = zfgn_NonemployeeServiceBasedOptionsMember |  |  |  | 17,513 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares / us-gaap_AwardTypeAxis = zfgn_NonemployeeServiceBasedOptionsMember |  |  |
Nonemployee Performance Based Options [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Number of outstanding unvested shares held by nonemployees | 796 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares / us-gaap_AwardTypeAxis = zfgn_NonemployeePerformanceBasedOptionsMember |  |  |  | 796 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares / us-gaap_AwardTypeAxis = zfgn_NonemployeePerformanceBasedOptionsMember |  |  |
Employee Stock Option [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Stock options granted |  |  |  |  | 527,973 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember |  |  |
Consultant [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Stock options granted |  |  |  |  | 1,592 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross / us-gaap_AwardTypeAxis = zfgn_ConsultantMember |  |  |
2006 Stock Option Plan [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Percentage of exercise price per share of stock options in fair market value of share of common stock |  | 100.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent / us-gaap_PlanNameAxis = zfgn_TwoThousandSixStockOptionPlanMember |  |  | 100.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent / us-gaap_PlanNameAxis = zfgn_TwoThousandSixStockOptionPlanMember |  |  |
Shares vesting period under stock options plan |  | 4 years |  |  |  |  |  |
Shares expiration period under stock option plan |  | 10 years |  |  |  |  |  |
Number of shares of common stock that could be issued under stock option plan | 1,889,150 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_PlanNameAxis = zfgn_TwoThousandSixStockOptionPlanMember | 1,889,150 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_PlanNameAxis = zfgn_TwoThousandSixStockOptionPlanMember |  |  | 1,889,150 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_PlanNameAxis = zfgn_TwoThousandSixStockOptionPlanMember |  |  |
Common stock available for issuance |  | 371,244 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / us-gaap_PlanNameAxis = zfgn_TwoThousandSixStockOptionPlanMember |  |  |  |  |  |
Shares of common stock reserved for issuance | 168,221 us-gaap_CommonStockCapitalSharesReservedForFutureIssuance / us-gaap_PlanNameAxis = zfgn_TwoThousandSixStockOptionPlanMember |  |  |  | 168,221 us-gaap_CommonStockCapitalSharesReservedForFutureIssuance / us-gaap_PlanNameAxis = zfgn_TwoThousandSixStockOptionPlanMember |  |  |
2006 Stock Option Plan [Member] | Maximum [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Shares vesting period under stock options plan |  | 10 years |  |  | 10 years |  |  |
2014 Stock Option and Incentive Plan [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Shares of common stock reserved for issuance | 2,168,221 us-gaap_CommonStockCapitalSharesReservedForFutureIssuance / us-gaap_PlanNameAxis = zfgn_TwoThousandFourteenStockOptionMember |  |  |  | 2,168,221 us-gaap_CommonStockCapitalSharesReservedForFutureIssuance / us-gaap_PlanNameAxis = zfgn_TwoThousandFourteenStockOptionMember |  |  |
Percentage of outstanding shares of common stock |  |  |  |  | 4.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingStockMaximum / us-gaap_PlanNameAxis = zfgn_TwoThousandFourteenStockOptionMember |  |  |
2014 Employee Stock Purchase Plan [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Common stock available for issuance | 265,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / us-gaap_PlanNameAxis = zfgn_TwoThousandAndFourteenEmployeeStockPurchasePlanMember |  |  |  | 265,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / us-gaap_PlanNameAxis = zfgn_TwoThousandAndFourteenEmployeeStockPurchasePlanMember |  |  |
Shares of common stock reserved for issuance |  |  |  |  |  |  |  | 265,000 us-gaap_CommonStockCapitalSharesReservedForFutureIssuance / us-gaap_PlanNameAxis = zfgn_TwoThousandAndFourteenEmployeeStockPurchasePlanMember
Commencement date of first offering period |  |  |  |  |  | Sep. 01, 2014 |  |
2014 Employee Stock Purchase Plan [Member] | Maximum [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Purchase price of closing market price of the company's common stock offering period |  |  |  |  |  | 85.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPriceOfferingDate / us-gaap_PlanNameAxis = zfgn_TwoThousandAndFourteenEmployeeStockPurchasePlanMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |

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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The cash inflow associated with the amount received from holders exercising their stock options. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately. + 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 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 3: 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 + Details
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- Definition Discount rate from fair value on offering date that participants pay for shares. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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