Income Taxes - Effect of Temporary Differences and Carryforwards (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 47,444	$ 31,989
Research and development tax credit carryforwards	2,332	1,654
Other	4,147us-gaap_DeferredTaxAssetsOther	2,369us-gaap_DeferredTaxAssetsOther
Total deferred tax assets	53,923us-gaap_DeferredTaxAssetsGross	36,012us-gaap_DeferredTaxAssetsGross
Less: Valuation allowance	(53,923)	(36,012)	(25,047)	(19,153)
Net deferred tax assets	$ 0us-gaap_DeferredTaxAssetsNet	$ 0us-gaap_DeferredTaxAssetsNet

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.

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

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