Share Based Compensation - Option valuation (Details) - Stock options	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Employees and directors
Assumptions to determine fair value of options granted on weighted average basis:
Risk-free interest rate	2.93%	1.90%	2.80%	1.97%
Expected term (in years)	6 years 3 months	6 years	6 years 4 months 28 days	6 years
Expected volatility	74.37%	73.88%	74.90%	74.23%
Non-employees
Assumptions to determine fair value of options granted on weighted average basis:
Risk-free interest rate	2.94%	1.97%	2.90%	1.97%
Expected term (in years)	7 years 15 days	8 years 10 months 2 days	7 years 5 months 16 days	8 years 10 months 2 days
Expected volatility	74.49%	77.93%	74.07%	77.93%

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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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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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