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Stock-Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended		12 Months Ended
	Mar. 31, 2021	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options, Outstanding, Beginning Balance	4,614,323	4,614,323
Granted		1,356,700
Exercised		(22,500)
Forfeited		(260,053)
Expired		(359,499)
Options, Outstanding, Ending Balance		5,328,971		4,614,323
Exercisable, Ending Balance		4,152,055
Weighted Average Exercise Price, Beginning Balance	$ 9.78	$ 9.78
Granted, Weighted Average Exercise Price		2.68	$ 2.08
Exercised, Weighted Average Exercise Price		2.11
Forfeited, Weighted Average Exercise Price		4.42
Expired, Weighted Average Exercise Price		12.38
Weighted Average Exercise Price, Ending Balance		8.09		$ 9.78
Exercisable, Weighted Average Exercise Price		$ 9.66
Outstanding, Ending balance, Weighted Average Remaining Contractual Term		6 years 7 months 6 days		6 years 9 months 18 days
Exercisable Ending Balance, Weighted Average Remaining Contractual Term		5 years 10 months 24 days
Outstanding, Intrinsic Value, Ending Balance				$ 2,949
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Accelerated vesting, number of shares	1,535,578
Unrecognized compensation cost related to nonvested stock-based compensation		$ 1,600
Weighted average remaining period over which unrecognized compensation expense will be recognized		2 years 1 month 6 days