Summary of Significant Accounting Policies - Summary of Stock-Based Compensation Assumptions (Details) - Stock Options	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employees and Directors
Summary of assumptions used to estimate the fair value of stock options
Risk-free interest rate	2.73%	2.20%	0.91%
Expected volatility	72.00%	73.00%	76.00%
Expected life in years	6 years	6 years	5 years
Dividend yield	0.00%	0.00%	0.00%
Forfeiture rate			7.00%
Non Employee
Summary of assumptions used to estimate the fair value of stock options
Risk-free interest rate (minimum)	2.60%	2.40%	2.30%
Risk-free interest rate (maximum)	3.00%	2.60%	2.50%
Dividend yield	0.00%	0.00%	0.00%
Non Employee \| Minimum
Summary of assumptions used to estimate the fair value of stock options
Expected volatility	68.00%	97.00%	71.00%
Expected life in years	9 years	9 years	7 years
Non Employee \| Maximum
Summary of assumptions used to estimate the fair value of stock options
Expected volatility	101.00%	102.00%	104.00%
Expected life in years	10 years	10 years	10 years

Assumptions used in estimation of fair value of stock options.

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- Definition

Forfeiture rate.

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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

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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- 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.

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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
-Subparagraph (f)(2)(i)
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-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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