Stock-Based Compensation - Assumptions Used in Determining Fair Value of Stock Options (Details) - $ / shares	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018
Summary of Significant Accounting Policies
Options to purchase common stock granted to employees and directors	610,002	569,199
Average risk free interest rate	2.30%	2.20%
Dividend rate	0.00%	0.00%
Expected lives (years)	4 years	3 years 10 months 24 days
Expected volatility	82.80%	75.60%
Weighted average exercise price of options granted during the period (per share)	$ 3.02	$ 17.42

No definition available.

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- Definition

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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- Definition

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

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- Definition

Net number of share options (or share units) granted during the period.

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- Definition

Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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No definition available.

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- Definition

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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-Name Accounting Standards Codification
-Topic 718
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