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Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) |  | 9 Months Ended | 167 Months Ended |  |
--- | --- | --- | --- | --- | ---
Jul. 02, 2018 | Sep. 30, 2019 | Sep. 30, 2019 | Dec. 31, 2018 | Dec. 29, 2017
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items] |  |  |  |  |
Entity incorporation date |  | Nov. 22, 2005 |  |  |
Restructuring and other attrition, percentage of workforce reduction |  | 70.00% |  |  |
Accumulated deficit |  | $ 389,100,000 | $ 389,100,000 | $ 350,945,000 |
Net proceeds from sale of securities |  |  | 397,900,000 |  |
Credit facility default amount |  | 300,000 | 300,000 |  |
Total cash, cash equivalents and marketable securities |  | 82,000,000 | 82,000,000 |  |
Term Loan [Member] |  |  |  |  |
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items] |  |  |  |  |
Aggregate principal balance |  | 16,400,000 | 16,400,000 |  | $ 20,000,000
Loan agreement minimum liquidity required |  | $ 17,200,000 | $ 17,200,000 | $ 21,000,000 |
Public Offering [Member] |  |  |  |  |
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items] |  |  |  |  |
Issuance of common stock | 9,200,000 |  |  |  |
Share price per share | $ 7.50 |  |  |  |
Net proceeds from public offering, after underwriting discount, commission and offering expenses | $ 64,600,000 |  |  |  |

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