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Fair Value of Financial Instruments - (Details) - USD ($) | Sep. 30, 2019 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Fair value of the Company's financial assets |  |  |
Marketable securities | $ 174,147,000 |  |
Assets transferred from level 1 to level 2 | 0 | $ 0 | $ 0
Assets transferred from level 2 to level 1 | 0 | 0 | 0
Carrying Amount |  |  |
Fair value of the Company's financial assets |  |  |
Money market funds (cash equivalents) |  | 5,000,000 | 1,000,000
Total financial assets | 179,670,000 |  |
Fair Value |  |  |
Fair value of the Company's financial assets |  |  |
Money market funds (cash equivalents) |  | 5,000,000 | 1,000,000
Total financial assets | 179,612,000 |  |
Fair Value | Quoted Prices in Active Markets (Level 1) |  |  |
Fair value of the Company's financial assets |  |  |
Money market funds (cash equivalents) |  | 5,000,000 | $ 1,000,000
Total financial assets | 5,465,000 |  |
Fair Value | Significant Other Observable Inputs (Level 2) |  |  |
Fair value of the Company's financial assets |  |  |
Total financial assets | 174,147,000 |  |
Money market funds (cash equivalents) | Carrying Amount |  |  |
Fair value of the Company's financial assets |  |  |
Money market funds (cash equivalents) | 5,465,000 | 5,000,000 |
Money market funds (cash equivalents) | Fair Value |  |  |
Fair value of the Company's financial assets |  |  |
Money market funds (cash equivalents) | 5,465,000 | 5,000,000 |
Money market funds (cash equivalents) | Fair Value | Quoted Prices in Active Markets (Level 1) |  |  |
Fair value of the Company's financial assets |  |  |
Money market funds (cash equivalents) | 5,465,000 | $ 5,000,000 |
U.S. treasury securities | Carrying Amount |  |  |
Fair value of the Company's financial assets |  |  |
Marketable securities | 19,982,000 |  |
U.S. treasury securities | Fair Value |  |  |
Fair value of the Company's financial assets |  |  |
Marketable securities | 19,983,000 |  |
U.S. treasury securities | Fair Value | Significant Other Observable Inputs (Level 2) |  |  |
Fair value of the Company's financial assets |  |  |
Marketable securities | 19,983,000 |  |
Agency bonds | Carrying Amount |  |  |
Fair value of the Company's financial assets |  |  |
Marketable securities | 47,178,000 |  |
Agency bonds | Fair Value |  |  |
Fair value of the Company's financial assets |  |  |
Marketable securities | 47,161,000 |  |
Agency bonds | Fair Value | Significant Other Observable Inputs (Level 2) |  |  |
Fair value of the Company's financial assets |  |  |
Marketable securities | 47,161,000 |  |
Corporate bonds | Carrying Amount |  |  |
Fair value of the Company's financial assets |  |  |
Marketable securities | 107,045,000 |  |
Corporate bonds | Fair Value |  |  |
Fair value of the Company's financial assets |  |  |
Marketable securities | 107,003,000 |  |
Corporate bonds | Fair Value | Significant Other Observable Inputs (Level 2) |  |  |
Fair value of the Company's financial assets |  |  |
Marketable securities | $ 107,003,000 |  |

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