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.

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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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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.

+ References

No definition available.

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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.

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No definition available.

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

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Stock-Based Compensation - Assumptions Used in Estimation of Fair Value of Stock (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Performance Based Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk Free Interest Rate				1.74%
Dividend yield				0.00%
Volatility				99.11%
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum	1.20%	0.70%	0.40%
Risk-free interest rate, maximum	2.60%	2.20%	1.10%
Dividend yield	0.00%	0.00%	0.00%
Volatility, minimum	52.20%	52.20%	46.00%
Volatility, maximum	171.60%	203.80%	267.80%
2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average Black-Scholes fair value of stock options and SARs granted	$ 4.98	$ 34.80	$ 21.20
Risk-free interest rate, minimum	1.50%	2.30%	1.60%
Risk-free interest rate, maximum	2.60%	3.10%	2.30%
Dividend yield	0.00%	0.00%	0.00%
Volatility, minimum	105.40%	93.30%	88.90%
Volatility, maximum	134.10%	115.60%	114.10%
Minimum [Member] \| Performance Based Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average Black-Scholes fair value of stock options and SARs granted			$ 14.80
Expected term (in years)			1 year 4 months 6 days
Minimum [Member] \| Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average Black-Scholes fair value of stock options and SARs granted	$ 2.57	$ 7.20	$ 9.00
Expected term (in years)	6 months	6 months	6 months
Minimum [Member] \| 2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	3 years 10 months 24 days	4 years 1 month 6 days	4 years 1 month 6 days
Maximum [Member] \| Performance Based Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average Black-Scholes fair value of stock options and SARs granted			$ 18.40
Expected term (in years)			3 years 6 months
Maximum [Member] \| Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average Black-Scholes fair value of stock options and SARs granted	$ 35.00	$ 70.64	$ 109.40