Stock-Based Compensation - Valuation (Details) - Stock options	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Black-Scholes option with following assumptions:
Expected stock price volatility, minimum (as a percent)	91.00%	90.00%	73.00%
Expected stock price volatility, maximum (as a percent)	98.00%	91.00%	85.00%
Expected option term	6 years	6 years	6 years
Risk-free interest rate, minimum (as a percent)	0.50%	1.60%	2.80%
Risk-free interest rate, maximum (as a percent)	0.70%	2.50%	3.10%
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%

No definition available.

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- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

+ References

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- Definition

The estimated measure of the maximum 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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No definition available.

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- Definition

The estimated measure of the minimum 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.

+ References

No definition available.

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- Definition

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

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No definition available.

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- Definition

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

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No definition available.

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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.

+ References

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