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Stock-based Compensation - Summary of Valuation Assumptions Used (Details)	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants
Risk-free interest rate	2.60%	2.60%
Volatility	63.30%	65.00%
Expected term (in years)	6 years 21 days	6 years 1 month 9 days

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- References

No definition available.

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- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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