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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
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Tax Disclosure [Abstract]
Valuation allowance as of beginning of year	$ 24,006us-gaap_DeferredTaxAssetsValuationAllowance	$ 18,744us-gaap_DeferredTaxAssetsValuationAllowance	$ 12,748us-gaap_DeferredTaxAssetsValuationAllowance
Decreases recorded as benefit to income tax provision	0zfgn_ValuationAllowanceDeferredTaxAssetChangeInAmountDecrease	0zfgn_ValuationAllowanceDeferredTaxAssetChangeInAmountDecrease	0zfgn_ValuationAllowanceDeferredTaxAssetChangeInAmountDecrease
Increases recorded to income tax provision	6,351zfgn_ValuationAllowanceDeferredTaxAssetChangeInAmountIncrease	5,262zfgn_ValuationAllowanceDeferredTaxAssetChangeInAmountIncrease	5,996zfgn_ValuationAllowanceDeferredTaxAssetChangeInAmountIncrease
Valuation allowance as of end of year	$ 30,357us-gaap_DeferredTaxAssetsValuationAllowance	$ 24,006us-gaap_DeferredTaxAssetsValuationAllowance	$ 18,744us-gaap_DeferredTaxAssetsValuationAllowance

X

- 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

X

- Details

Name:	us-gaap_IncomeTaxDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- 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

X

- 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