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Summary of Significant Accounting Policies - Summary of Fair Value Measurement Financial Asset and Liabilities (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Assets [Abstract] |  |
Cash and cash equivalents | $ 146,200 | $ 62,500
Securities |  | 93,394
Common stock warrants | 1,890 |
Total assets measured at fair value | 160,849 | 155,910
Money market funds |  |
Assets [Abstract] |  |
Cash and cash equivalents | 46,257 | 61,512
U.S Treasury securities |  |
Assets [Abstract] |  |
Marketable securities |  | 3,989
Securities | 12,488 |
Government-sponsored enterprises |  |
Assets [Abstract] |  |
Marketable securities |  | 11,991
Corporate debt securities |  |
Assets [Abstract] |  |
Marketable securities |  | 78,418
Securities | 100,214 |
Quoted Prices in Active Markets for Identical Assets, Level 1 |  |
Assets [Abstract] |  |
Common stock warrants | 0 |
Total assets measured at fair value | 46,257 | 61,512
Quoted Prices in Active Markets for Identical Assets, Level 1 | Money market funds |  |
Assets [Abstract] |  |
Cash and cash equivalents | 46,257 | 61,512
Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S Treasury securities |  |
Assets [Abstract] |  |
Marketable securities |  | 0
Securities | 0 |
Quoted Prices in Active Markets for Identical Assets, Level 1 | Government-sponsored enterprises |  |
Assets [Abstract] |  |
Marketable securities |  | 0
Quoted Prices in Active Markets for Identical Assets, Level 1 | Corporate debt securities |  |
Assets [Abstract] |  |
Marketable securities |  | 0
Securities | 0 |
Significant Other Observable Inputs, Level 2 |  |
Assets [Abstract] |  |
Common stock warrants | 0 |
Total assets measured at fair value | 112,702 | 94,398
Significant Other Observable Inputs, Level 2 | Money market funds |  |
Assets [Abstract] |  |
Cash and cash equivalents | 0 | 0
Significant Other Observable Inputs, Level 2 | U.S Treasury securities |  |
Assets [Abstract] |  |
Marketable securities |  | 3,989
Securities | 12,488 |
Significant Other Observable Inputs, Level 2 | Government-sponsored enterprises |  |
Assets [Abstract] |  |
Marketable securities |  | 11,991
Significant Other Observable Inputs, Level 2 | Corporate debt securities |  |
Assets [Abstract] |  |
Marketable securities |  | 78,418
Securities | 100,214 |
Significant Unobservable Inputs, Level 3 |  |
Assets [Abstract] |  |
Common stock warrants | 1,890 |
Total assets measured at fair value | 1,890 | 0
Significant Unobservable Inputs, Level 3 | Money market funds |  |
Assets [Abstract] |  |
Cash and cash equivalents | 0 | 0
Significant Unobservable Inputs, Level 3 | U.S Treasury securities |  |
Assets [Abstract] |  |
Marketable securities |  | 0
Securities | 0 |
Significant Unobservable Inputs, Level 3 | Government-sponsored enterprises |  |
Assets [Abstract] |  |
Marketable securities |  | 0
Significant Unobservable Inputs, Level 3 | Corporate debt securities |  |
Assets [Abstract] |  |
Marketable securities |  | $ 0
Securities | $ 0 |

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