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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - STOCK OPTIONS AND EMPLOYEE STOCK PURCHASE PLAN (Details)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Stock Options
Stock-Based Compensation
Vesting period of awards	4 years
Assumptions used to estimate fair value of awards
Dividend yield (as a percent)	0.00%	0.00%
Expected volatility range	78.10%
Expected volatility range, minimum (as a percent)		92.50%
Expected volatility range, maximum (as a percent)		98.00%
Risk-free interest rate range, minimum (as a percent)	1.42%	2.24%
Risk-free interest rate range, maximum (as a percent)	1.62%	2.56%
Expected term range	5 years 3 months
Stock Options | Minimum
Assumptions used to estimate fair value of awards
Expected term range		5 years 3 months
Stock Options | Maximum
Assumptions used to estimate fair value of awards
Expected term range		6 years 5 months 8 days
Employee Stock Purchase Plan
Assumptions used to estimate fair value of awards
Dividend yield (as a percent)	0.00%	0.00%
Expected volatility range, minimum (as a percent)	47.80%	133.30%
Expected volatility range, maximum (as a percent)	56.40%	165.30%
Risk-free interest rate range, minimum (as a percent)	1.56%	2.56%
Risk-free interest rate range, maximum (as a percent)	1.57%	2.63%
Employee Stock Purchase Plan | Minimum
Assumptions used to estimate fair value of awards
Expected term range	6 months	6 months
Employee Stock Purchase Plan | Maximum
Assumptions used to estimate fair value of awards
Expected term range	12 months	12 months

X

- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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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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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.

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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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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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