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Income Taxes - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Income Taxes Disclosure [Line Items]      
Income tax benefit $ 0us-gaap_IncomeTaxExpenseBenefit $ 0us-gaap_IncomeTaxExpenseBenefit $ 0us-gaap_IncomeTaxExpenseBenefit
Net operating loss carryforwards, additional amount 16zfgn_OperatingLossCarryforwardsAdditional    
Uncertain tax position 0us-gaap_LiabilityForUncertainTaxPositionsCurrent    
Gross deferred tax asset 30,357us-gaap_DeferredTaxAssetsGross 24,006us-gaap_DeferredTaxAssetsGross  
Unrecognized tax benefits   0us-gaap_UnrecognizedTaxBenefits  
Domestic Tax Authority [Member]      
Income Taxes Disclosure [Line Items]      
Net operating loss carryforwards 10,548us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember		    
Net operating loss carryforwards begin to expire 2026    
Research and development tax credit carryforwards 4,706us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember		    
Research and development tax credit carryforwards begin to expire 2026    
State and Local Jurisdiction [Member]      
Income Taxes Disclosure [Line Items]      
Net operating loss carryforwards 8,226us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		    
Net operating loss carryforwards begin to expire 2014    
Research and development tax credit carryforwards $ 1,322us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		    
Research and development tax credit carryforwards begin to expire 2021