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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - STOCK OPTIONS AND EMPLOYEE STOCK PURCHASE PLAN (Details)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
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, minimum (as a percent)	82.10%	88.40%
Expected volatility range, maximum (as a percent)	83.20%	89.20%
Risk-free interest rate range, minimum (as a percent)	2.55%	1.98%
Risk-free interest rate range, maximum (as a percent)	2.94%	1.99%
Expected term	5 years 3 months	5 years 6 months
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)	43.70%	57.70%
Expected volatility range, maximum (as a percent)	47.50%	64.10%
Risk-free interest rate range, minimum (as a percent)	1.53%	0.45%
Risk-free interest rate range, maximum (as a percent)	1.76%	0.89%
Employee Stock Purchase Plan | Minimum
Assumptions used to estimate fair value of awards
Expected term	6 months	6 months
Employee Stock Purchase Plan | Maximum
Assumptions used to estimate fair value of awards
Expected term	12 months	12 months

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- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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

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