Stockholders' Equity (Assumptions Used to Estimate Fair Value of Warrants Granted Using Black-Scholes Option Pricing Model) (Details) - $ / shares	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Granted to employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average fair value of underlying stock at grant date	$ 2.38	$ 1.48	$ 1.56
Dividend yields
Expected volatility, minimum	68.00%	45.00%	57.00%
Expected volatility, maximum	87.00%	69.00%	69.00%
Risk-free interest rates, minimum	2.01%	1.45%	0.94%
Risk-free interest rates, maximum	2.74%	1.94%	1.93%
Weighted-average grant date fair value	$ 1.72	$ 1.23	$ 1.35
Granted to employees [Member] \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected lives (in years)	3 years	3 years	3 years
Granted to employees [Member] \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected lives (in years)	5 years 6 months	5 years 10 months 14 days	5 years 6 months
Granted to non-employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average fair value of underlying stock at grant date	$ 3.37	$ 1.56	$ 1.51
Dividend yields
Expected volatility, minimum	73.00%	68.00%	69.00%
Expected volatility, maximum	76.00%	72.00%	70.00%
Risk-free interest rates, minimum	2.46%	2.36%	1.73%
Risk-free interest rates, maximum	2.81%	2.44%	1.83%
Expected lives (in years)	10 years	10 years	10 years
Weighted-average grant date fair value	$ 3.36	$ 1.55	$ 1.50

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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- Definition

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.

+ References

No definition available.

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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.

+ References

No definition available.

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- Definition

The grant-date intrinsic value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

+ References

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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

+ 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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