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Stock-Based Compensation - Fair Value of Employee Option Grant Estimated on Date of Grant (Details) - Equity Option - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award
Estimated weighted-average fair value (USD per share)	$ 8.19	$ 9.93	$ 10.32
Weighted Average
Weighted-average assumptions
Expected volatility	77.00%	80.00%	78.00%
Expected term, in years	6 years	6 years	6 years
Risk-free interest rate	1.06%	2.41%	2.68%
Expected dividend yield	0.00%	0.00%	0.00%

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

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

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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.

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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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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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