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

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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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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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Number of shares authorized for issuance under share-based payment arrangement.

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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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Description of terms of award under share-based payment arrangement.

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

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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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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Stock-Based Compensation - Fair value assumptions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Feb. 28, 2018
Stock-Based Compensation
Authorized shares	3,389,956
Fair value of grants per share			$ 6.88
Options
Stock-Based Compensation
Terms of award	ten
Vesting period	4 years
Weighted-average grant-date fair value of options (in dollars per share)	$ 9.68	$ 7.64
Expected stock price volatility, minimum (as a percent)	78.05%
Expected stock price volatility, maximum (as a percent)	79.48%
Risk-free rate of interest, minimum (as a percent)	1.51%
Risk-free rate of interest, maximum (as a percent)	2.53%
Expected dividend yield (as a percent)	0.00%
Options \| Minimum
Stock-Based Compensation
Exercise price per share (in dollars per share)	$ 4.14
Fair value of grants per share	$ 2.73
Expected Term (years)	5 years
Options \| Maximum
Stock-Based Compensation
Exercise price per share (in dollars per share)	$ 11.28
Fair value of grants per share	$ 8.17
Expected Term (years)	7 years