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Summary of Changes in the Valuation Allowance for Deferred Tax Assets (Detail) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Income Tax Disclosure [Abstract] |  |  |
Valuation allowance as of beginning of year | $ 24,006 us-gaap_DeferredTaxAssetsValuationAllowance | $ 18,744 us-gaap_DeferredTaxAssetsValuationAllowance | $ 12,748 us-gaap_DeferredTaxAssetsValuationAllowance
Decreases recorded as benefit to income tax provision | 0 zfgn_ValuationAllowanceDeferredTaxAssetChangeInAmountDecrease | 0 zfgn_ValuationAllowanceDeferredTaxAssetChangeInAmountDecrease | 0 zfgn_ValuationAllowanceDeferredTaxAssetChangeInAmountDecrease
Increases recorded to income tax provision | 6,351 zfgn_ValuationAllowanceDeferredTaxAssetChangeInAmountIncrease | 5,262 zfgn_ValuationAllowanceDeferredTaxAssetChangeInAmountIncrease | 5,996 zfgn_ValuationAllowanceDeferredTaxAssetChangeInAmountIncrease
Valuation allowance as of end of year | $ 30,357 us-gaap_DeferredTaxAssetsValuationAllowance | $ 24,006 us-gaap_DeferredTaxAssetsValuationAllowance | $ 18,744 us-gaap_DeferredTaxAssetsValuationAllowance

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- Definition Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32537-109319 + Details
Name: | us-gaap_DeferredTaxAssetsValuationAllowance
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Valuation Allowance Deferred Tax Asset Change In Amount Decrease + References No definition available. + Details
Name: | zfgn_ValuationAllowanceDeferredTaxAssetChangeInAmountDecrease
Namespace Prefix: | zfgn_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Valuation Allowance Deferred Tax Asset Change In Amount Increase + References No definition available. + Details
Name: | zfgn_ValuationAllowanceDeferredTaxAssetChangeInAmountIncrease
Namespace Prefix: | zfgn_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration