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Summary of Significant Accounting Policies - Additional Information (Detail) | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |
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Jul. 31, 2015 | Dec. 31, 2018 USD ($) | Sep. 30, 2018 USD ($) | Jun. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2018 USD ($) Segment | Dec. 31, 2017 USD ($) | Jan. 01, 2018 USD ($) | Aug. 28, 2017
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue, practical expedient, incremental cost of obtaining contract [true/false] |  |  |  |  |  |  |  |  |  | true |  |  |
Revenue, practical expedient, remaining performance obligation, description |  |  |  |  |  |  |  |  |  | The Company recognizes as an asset the incremental costs of obtaining a contract with a customer if the costs are expected to be recovered. As a practical expedient, the Company recognizes the incremental costs of obtaining a contract as an expense when incurred if the amortization period of the asset that we otherwise would have recognized is one year or less. To date, the Company has not incurred any incremental costs of obtaining a contract with a customer |  |  |
Cash equivalents maturity |  |  |  |  |  |  |  |  |  | three months or less |  |  |
Cash equivalents |  | $ 300,000 |  |  |  | $ 32,700,000 |  |  |  | $ 300,000 | $ 32,700,000 |  |
Cash held in a separate restricted bank account |  | 1,097,000 |  |  |  | 1,097,000 |  |  |  | 1,097,000 | 1,097,000 |  |
Transfers of assets and liabilities between Level 1, Level 2, or Level 3 |  |  |  |  |  |  |  |  |  | 0 | 0 |  |
Impairment charge |  |  |  |  |  |  |  |  |  | $ 0 |  |  |
Lease expiration date | 2021-04 |  |  |  |  |  |  |  |  | 2017-07 |  |  |
Incentive unit liquidation value |  | 0 |  |  |  |  |  |  |  | $ 0 |  |  |
Description of tax benefit likely to be realized upon settlement |  |  |  |  |  |  |  |  |  | greater than 50% |  |  |
Exchange ratio of common stock |  |  |  |  |  |  |  |  |  |  |  |  | 0.5532
Number of operating segment | Segment |  |  |  |  |  |  |  |  |  | 1 |  |  |
Revenue |  | 111,000 | $ 1,801,000 | $ 254,000 | $ 354,000 | 111,000 | $ 111,000 | $ 2,111,000 | $ 111,000 | $ 2,520,000 | 2,444,000 |  |
ASC 606 |  |  |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative-effect decrease to opening accumulated deficit |  |  |  |  |  |  |  |  |  |  |  | $ 300,000 |
Deferred revenue |  | 300,000 |  |  |  |  |  |  |  | 300,000 |  |  |
Revenue |  | 2,500,000 |  |  |  |  |  |  |  |  |  |  |
ASC 606 | Impact as Result of Adoption of ASC 606 |  |  |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred revenue |  | (300,000) |  |  |  |  |  |  |  | (300,000) |  |  |
ASC 606 | Balances without Adoption of ASC 606 |  |  |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred revenue |  | 700,000 |  |  |  |  |  |  |  | 700,000 |  |  |
Revenue |  | 2,400,000 |  |  |  |  |  |  |  |  |  |  |
Bank Account |  |  |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Cash held in a separate restricted bank account |  | 50,000 |  |  |  | $ 50,000 |  |  |  | 50,000 | $ 50,000 |  |
Letter of Credit |  |  |  |  |  |  |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Cash |  | $ 1,000,000 |  |  |  |  |  |  |  | $ 1,000,000 |  |  |

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