Common Stock and Stockholders' Equity - Assumptions Used to Estimate Fair Value of Stock Options and Performance Stock and Employee Stock Purchase Plan (Details)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Stock options
Weighted average assumptions
Risk-free interest rate	1.70%	2.70%
Volatility	94.60%	87.80%
Dividend yield	0.00%	0.00%
Expected term (years)	6 years 1 month 6 days	6 years 1 month 6 days
Performance stock options
Weighted average assumptions
Risk-free interest rate	2.60%	2.70%
Volatility	93.80%	87.40%
Dividend yield	0.00%	0.00%
Expected term (years)	6 years 1 month 6 days	5 years 7 months 28 days
Employee Stock Purchase Plan
Weighted average assumptions
Risk-free interest rate	2.30%	1.90%
Volatility	110.50%	100.60%
Dividend yield	0.00%	0.00%
Expected term (years)	6 months	6 months

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

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
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-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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