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Stock Incentive Plans - Stock-Based Compensation Expense (Details) - Stock options	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Assumptions used to calculate fair value of stock options
Expected dividend yield	0.00%	0.00%
Minimum
Assumptions used to calculate fair value of stock options
Weighted-average expected volatility	75.51%	68.90%
Expected term (in years)	5 years 6 months	5 years 6 months
Risk-free interest rate	0.37%	1.39%
Maximum
Assumptions used to calculate fair value of stock options
Weighted-average expected volatility	82.08%	110.50%
Expected term (in years)	6 years 3 months	10 years
Risk-free interest rate	1.51%	2.69%

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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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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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