Stockholders’ (Deficit) Equity and Stock-Based Compensation - Option reward activity (Details) - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Number of options outstanding
Beginning balance (in shares)	11,464,764	17,558,731	17,558,731	12,890,557
Options granted (in shares)		0	7,248,934	5,319,061
Options exercised (in shares)		(139,423)	(12,796,353)	(259,070)
Options forfeited (in shares)		(335,424)	(544,526)	(281,669)
Options expired (in shares)		(38,651)	(2,022)	(110,148)
Ending balance (in shares)	10,951,266		11,464,764	17,558,731	12,890,557
Weighted-average exercise price per share
Beginning balance (in dollars per share)	$ 2.66	$ 0.93	$ 0.93	$ 0.76
Options granted (in dollars per share)		0	3.93	1.05
Options exercised (in dollars per share)		1.04	0.79	0.49
Options forfeited (in dollars per share)		5.04	2.53	0.97
Options expired (in dollars per share)		$ 0.83	0.73	0.76
Ending balance (in dollars per share)	$ 2.61		$ 2.66	$ 0.93	$ 0.76
Weighted-average remaining contractual life (years)	6 years 9 months 18 days		8 years 3 months 18 days	8 years 2 months 12 days	8 years 7 months 6 days
Aggregate intrinsic value	$ 12,532,000		$ 53,695,000	$ 30,881,000	$ 5,836,000
Options vested, Number of options outstanding (in shares)	5,466,700		4,830,158	1,947,361
Options exercisable, Number of options outstanding (in shares)	5,466,700		4,830,158	1,947,361
Options vested, Weighted-average exercise price per share (in dollars per share)	$ 1.54		$ 1.30	$ 0.63
Options exercisable, Weighted-average exercise price per share (in dollars per share)	$ 1.54		$ 1.30	$ 0.63
Options vested, Weighted-average remaining contractual life (years)	6 years 1 month 6 days		7 years 4 months 24 days	7 years
Options exercisable, Weighted-average remaining contractual life (years)	6 years 1 month 6 days		7 years 4 months 24 days	7 years
Options vested, Aggregate intrinsic value	$ 8,607,549		$ 29,176,000	$ 4,734,000
Options exercisable, Aggregate intrinsic value	$ 8,607,549		$ 29,176,000	$ 4,734,000

The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Number of fully vested and expected to vest exercisable options that may be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options of the plan that expired.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of share options (or share units) exercised during the current period.

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