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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		$ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	$ 12us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	$ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
Proceeds from exercise of stock options			13us-gaap_ProceedsFromStockOptionsExercised
Weighted average grant-date fair value of stock options granted		$ 1.83us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	$ 0.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	$ 1.07us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Unrecognized stock-based compensation cost	6,224us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized	1,334us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized			6,224us-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		16,726,699us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted					529,565us-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		17,513us-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		796us-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			10,615us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	24,548us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember			38,481us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Aggregate intrinsic value of restricted stock awards vested during the period		$ 38us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	$ 22us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	$ 22us-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	17,513us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares / us-gaap_AwardTypeAxis = zfgn_NonemployeeServiceBasedOptionsMember				17,513us-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	796us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares / us-gaap_AwardTypeAxis = zfgn_NonemployeePerformanceBasedOptionsMember				796us-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					527,973us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember
Consultant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted					1,592us-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	1,889,150us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_PlanNameAxis = zfgn_TwoThousandSixStockOptionPlanMember	1,889,150us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_PlanNameAxis = zfgn_TwoThousandSixStockOptionPlanMember			1,889,150us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_PlanNameAxis = zfgn_TwoThousandSixStockOptionPlanMember
Common stock available for issuance		371,244us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / us-gaap_PlanNameAxis = zfgn_TwoThousandSixStockOptionPlanMember
Shares of common stock reserved for issuance	168,221us-gaap_CommonStockCapitalSharesReservedForFutureIssuance / us-gaap_PlanNameAxis = zfgn_TwoThousandSixStockOptionPlanMember				168,221us-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	2,168,221us-gaap_CommonStockCapitalSharesReservedForFutureIssuance / us-gaap_PlanNameAxis = zfgn_TwoThousandFourteenStockOptionMember				2,168,221us-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	265,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / us-gaap_PlanNameAxis = zfgn_TwoThousandAndFourteenEmployeeStockPurchasePlanMember				265,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / us-gaap_PlanNameAxis = zfgn_TwoThousandAndFourteenEmployeeStockPurchasePlanMember
Shares of common stock reserved for issuance								265,000us-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]
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Aggregate number of common shares reserved for future issuance.

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Unrecognized cost of unvested share-based compensation awards.

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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.

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Gross number of share options (or share units) granted during the period.

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