Summary of Significant Accounting Policies - Recently Adopted Accounting Pronouncements (Details) - USD ($)
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12 Months Ended |
Jan. 01, 2018 |
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
| Selected Quarterly Financial Data |
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|
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| Additional paid-in capital |
|
$ 543,040,000
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$ 17,277,000
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| Accumulated deficit |
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$ (150,082,000)
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$ (60,979,000)
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| Change in the fair value of the unvested awards |
$ 100,000
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| Assumptions used to determine the fair value of outstanding awards granted |
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| Expected volatility |
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74.00%
|
75.60%
|
77.20%
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| Non-employees |
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| Assumptions used to determine the fair value of outstanding awards granted |
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| Weighted average grant-date fair value (per share) |
$ 5.88
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| Risk-free interest rate |
2.30%
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| Expected volatility |
74.00%
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| Expected term (in years) |
6 years 1 month 6 days
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| Common stock value |
$ 6.28
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| Restricted Common Stock | Non-employees |
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| Assumptions used to determine the fair value of outstanding awards granted |
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| Number of outstanding options |
4,767,014
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| Stock options | Non-employees |
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| Assumptions used to determine the fair value of outstanding awards granted |
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|
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| Number of outstanding options |
330,917
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| ASU 2018-07 |
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| Selected Quarterly Financial Data |
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| Additional paid-in capital |
$ 17,185,000
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|
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| Accumulated deficit |
(60,887,000)
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| Adjustments for New Accounting Principle, Early Adoption [Member] | ASU 2018-07 |
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| Selected Quarterly Financial Data |
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|
|
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| Additional paid-in capital |
(92,000)
|
|
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| Accumulated deficit |
$ 92,000
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