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Significant Accounting Policies (Details) - USD ($) $ in Millions | 6 Months Ended |
--- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Dec. 31, 2018
Share-based Compensation |  |  |
Expected term | 6 years 3 months | 6 years 3 months |
Level 1 |  |  |
Fair Value Measurements |  |  |
Carrying value of cash held in money market fund | $ 182.2 |  | $ 207.9
Level 2 |  |  |
Fair Value Measurements |  |  |
Assets measured at fair value levels 2 or 3 | 0.0 |  | 0.0
Liabilities measured at fair value levels 2 or 3 | 0.0 |  | 0.0
Level 3 |  |  |
Fair Value Measurements |  |  |
Assets measured at fair value levels 2 or 3 | 0.0 |  | 0.0
Liabilities measured at fair value levels 2 or 3 | $ 0.0 |  | $ 0.0
Maximum |  |  |
Share-based Compensation |  |  |
Expiration period | 10 years |  |

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