Stock-Based Compensation - Stock-based Valuation Information (Details)	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019
Employee Stock Option [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	1.30%	2.50%
Dividend yield	0.00%	0.00%
Volatility	74.50%	76.30%
Expected life	6 years 1 month 6 days	6 years 1 month 6 days
Board of Director Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	0.80%	1.90%
Dividend yield	0.00%	0.00%
Volatility	75.30%	74.30%
Expected life	5 years 8 months 12 days	6 years 3 months 18 days
2017 ESPP
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	1.00%	2.50%
Dividend yield	0.00%	0.00%
Volatility	71.90%	64.10%
Expected life	6 months	6 months

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

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- Definition

Rate of weighted-average expected volatility for award under share-based payment arrangement.

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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
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