Share-Based Compensation - Option Valuation (Details) - Stock options	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Employees
Assumptions to determine fair value of options granted on weighted average basis:
Risk-free interest rate	0.55%	2.07%	2.82%
Expected term (in years)	6 years 2 months 15 days	6 years 2 months 19 days	6 years 4 months 24 days
Expected volatility	80.81%	79.14%	75.04%
Non-employees
Assumptions to determine fair value of options granted on weighted average basis:
Risk-free interest rate	1.38%	2.91%
Expected term (in years)	2 years 9 months 7 days	7 years 4 months 6 days
Expected volatility	68.17%	74.18%

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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 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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