Summary of Significant Accounting Policies - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Liabilities
Expected volatility rate period	180 days
Minimum [Member]
Liabilities
Maximum volatility rate	40.00%
Common Stock Warrant Liabilities [Member]
Liabilities
Liabilities measured at fair value on a recurring basis	4,683	$ 5,093
Embedded Derivative Liabilities [Member]
Liabilities
Liabilities measured at fair value on a recurring basis	52,000	53,000
Money market fund shares [Member]
Assets
Assets measured at fair value on a recurring basis	18,125	8,021
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Common Stock Warrant Liabilities [Member]
Liabilities
Liabilities measured at fair value on a recurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Embedded Derivative Liabilities [Member]
Liabilities
Liabilities measured at fair value on a recurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Money market fund shares [Member]
Assets
Assets measured at fair value on a recurring basis	18,125	8,021
Significant Other Observable Inputs (Level 2) [Member] \| Common Stock Warrant Liabilities [Member]
Liabilities
Liabilities measured at fair value on a recurring basis	0	0
Significant Other Observable Inputs (Level 2) [Member] \| Embedded Derivative Liabilities [Member]
Liabilities
Liabilities measured at fair value on a recurring basis	0	0
Significant Other Observable Inputs (Level 2) [Member] \| Money market fund shares [Member]
Assets
Assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) [Member] \| Common Stock Warrant Liabilities [Member]
Liabilities
Liabilities measured at fair value on a recurring basis	4,683	5,093
Significant Unobservable Inputs (Level 3) [Member] \| Embedded Derivative Liabilities [Member]
Liabilities
Liabilities measured at fair value on a recurring basis	52,000	53,000
Significant Unobservable Inputs (Level 3) [Member] \| Money market fund shares [Member]
Assets
Assets measured at fair value on a recurring basis	0	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

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Measure of dispersion, in percentage terms (for instance, the standard deviation or variance), for a given stock price.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

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Fair Value Assumptions, Expected Volatility Rate, Period

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