Stockholders Deficit - Summary of Material Factors Incorporated in Black-Scholes Model in Estimating Fair Value of Options Granted (Details) - $ / shares	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum	2.59%	1.75%
Risk-free interest rate, maximum	2.82%	2.20%
Expected volatility, minimum	79.50%	58.50%
Expected volatility, maximum	85.80%	62.60%
Expected dividend yield	0.00%	0.00%
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum	1.88%	0.61%
Risk-free interest rate, maximum	2.07%	1.12%
Expected volatility, minimum	83.20%	68.00%
Expected volatility, maximum	85.80%	70.00%
Expected term	6 months	6 months
Expected dividend yield	0.00%	0.00%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	5 years 3 months 18 days	5 years 1 month 6 days
Minimum \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 3.26	$ 1.71
Minimum \| Common Stock \| Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 1.85	$ 2.11
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	5 years 6 months	6 years 3 months 18 days
Maximum \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 5.06	$ 3.46
Maximum \| Common Stock \| Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 4.00	$ 3.17

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 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.

+ 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.

+ References

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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- 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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- Definition

Price of a single share of a number of saleable stocks of a company.

+ References

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- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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