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Summary of Significant Accounting Policies - Summary of Fair Value Measurement Financial Asset and Liabilities (Detail) (USD $) | Sep. 30, 2013 | Dec. 31, 2012 | Dec. 31, 2011
--- | --- | --- | ---
Assets: |  |  |
Cash and cash equivalents | $ 7,522,029 | $ 18,695,197 | $ 31,049,050
Money market funds | 26,047,169 | 29,047,958 | 24,169,311
Restricted cash | 404,850 | 404,850 | 582,171
Total assets | 33,974,048 | 48,148,005 | 55,800,532
Liabilities: |  |  |
Preferred stock warrant liability | (679,296) | (52,947) | (203,642)
Quoted Prices in Active Markets for Identical Assets Level 1 [Member] |  |  |
Assets: |  |  |
Cash and cash equivalents | 7,522,029 | 18,695,197 | 31,049,050
Money market funds | 26,047,169 | 29,047,958 |
Restricted cash | 404,850 | 404,850 | 582,171
Total assets | 33,974,048 | 48,148,005 | 31,631,221
Liabilities: |  |  |
Preferred stock warrant liability |  |  |
Significant Other Observable Inputs Level 2 [Member] |  |  |
Assets: |  |  |
Cash and cash equivalents |  |  |
Money market funds |  |  | 24,169,311
Restricted cash |  |  |
Total assets |  |  | 24,169,311
Liabilities: |  |  |
Preferred stock warrant liability |  |  |
Significant Unobservable Inputs Level 3 [Member] |  |  |
Assets: |  |  |
Cash and cash equivalents |  |  |
Money market funds |  |  |
Restricted cash |  |  |
Total assets |  |  |
Liabilities: |  |  |
Preferred stock warrant liability | $ (679,296) | $ (52,947) | $ (203,642)

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + 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 (a) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash -URI http://asc.fasb.org/extlink&oid=6506951 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash Equivalents -URI http://asc.fasb.org/extlink&oid=6507016 + Details
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- Definition Fair value portion of warrants not settleable in cash classified as equity. + 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 (a) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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