Stock-Based Compensation - Assumptions Used in Estimation of Fair Value of Stock (Details) - $ / shares	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average Black-Scholes fair value of stock options granted	$ 8.50	$ 26.61	$ 23.43	$ 27.55
Dividend yield	0.00%	0.00%	0.00%	0.00%
Expected forfeiture rate	0.00%	0.00%	0.00%	0.00%
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.00%	0.00%	0.00%	0.00%
Expected forfeiture rate	0.00%	0.00%	0.00%	0.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum	1.81%	2.59%	1.81%	2.26%
Volatility, minimum	127.32%	113.68%	111.65%	113.98%
Expected term (in years)	4 years 15 days	4 years 1 month 6 days	4 years 15 days	4 years 1 month 6 days
Minimum [Member] \| Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average Black-Scholes fair value of stock options granted	$ 9.45	$ 13.21	$ 7.25	$ 8.99
Risk-free interest rate, minimum	1.34%	0.74%	1.20%	0.66%
Volatility, minimum	59.71%	59.84%	52.19%	59.84%
Expected term (in years)	6 months	6 months	6 months	6 months
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, maximum	2.33%	2.74%	2.55%	2.74%
Volatility, maximum	128.49%	114.24%	128.49%	114.12%
Expected term (in years)	4 years 18 days	4 years 1 month 10 days	4 years 6 months	4 years 1 month 21 days
Maximum [Member] \| Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average Black-Scholes fair value of stock options granted	$ 34.78	$ 70.64	$ 34.78	$ 70.64
Risk-free interest rate, maximum	2.50%	1.79%	2.50%	1.79%
Volatility, maximum	171.60%	203.83%	171.60%	203.83%
Expected term (in years)	2 years	2 years	2 years	2 years

Estimated rate of forfeitures.

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.

+ References

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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.

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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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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 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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- 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
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-SubTopic 10
-Section S99
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-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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