Income Taxes - Significant Components of the Company's Deferred Tax Assets (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets:
Net operating loss carryforwards	$ 11,262	$ 13,069
Deferred revenue	11,099us-gaap_DeferredTaxAssetsDeferredIncome	9,723us-gaap_DeferredTaxAssetsDeferredIncome
Research credits	1,260	1,734
Other	1,201us-gaap_DeferredTaxAssetsOther	475us-gaap_DeferredTaxAssetsOther
Gross deferred tax assets	24,822us-gaap_DeferredTaxAssetsGross	25,001us-gaap_DeferredTaxAssetsGross
Valuation allowance	(24,741)	(25,001)
Total deferred tax assets	81us-gaap_DeferredTaxAssetsNet
Deferred tax liabilities	(81)us-gaap_DeferredTaxLiabilities
Net deferred tax assets	$ 0us-gaap_DeferredTaxAssetsLiabilitiesNet	$ 0us-gaap_DeferredTaxAssetsLiabilitiesNet

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from deferred income.

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Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards, net of deferred tax liability attributable to taxable temporary differences.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible operating loss carryforwards.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible research tax credit carryforwards.

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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Amount of deferred tax liability attributable to taxable temporary differences net of deferred tax asset attributable to deductible temporary differences and carryforwards after valuation allowances.

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