Stock Based Compensation - Assumptions estimate the fair value of stock options (Details) - Stock Options - $ / shares	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock Based Compensation
Expected volatility, minimum	45.00%	47.17%	47.69%
Expected volatility, maximum	50.00%	55.47%	49.17%
Risk-free interest rate, minimum		1.57%	2.60%
Risk-free interest rate, maximum		2.64%	3.06%
Expected dividend yield	0.00%	0.00%	0.00%
Weighted average estimated fair value of stock options granted during the year	$ 5.06	$ 0.21	$ 0.11
Minimum
Stock Based Compensation
Risk-free interest rate, minimum	0.27%
Expected term (in years)	4 years 1 month 21 days	5 years	5 years 5 months 27 days
Maximum
Stock Based Compensation
Risk-free interest rate, maximum	1.44%
Expected term (in years)	6 years 3 months	6 years 10 months 10 days	6 years 1 month 17 days

Agreed-upon price for the exchange of the underlying asset relating to the share-based payment award.

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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 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.

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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.

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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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- 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.

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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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