Stock-Based Compensation - Service Based Stock Options (Details) - Stock option awards - service based awards - USD ($)	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Shares Underlying Options
Outstanding at the beginning of the period (in shares)	5,717,956	8,210,024	8,210,024
Granted (in shares)	1,872,162	2,252,344
Forfeited (in shares)	(385,278)	(440,026)
Expired (in shares)	(1,333,220)	(4,090,150)
Outstanding at the end of the period (in shares)	5,871,620	5,932,192	5,717,956	8,210,024
Exercisable at end of period (in shares)	2,539,438	2,889,218
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 4.55	$ 7.06	$ 7.06
Granted (in dollars per share)	0.89	1.95
Forfeited (in dollars per share)	2.30	3.96
Expired (in dollar per share)	7.16	8.24
Outstanding at the end of the period (in dollars per share)	2.95	4.54	$ 4.55	$ 7.06
Exercisable at end of the period (in dollars per share)	$ 4.96	$ 6.95
Weighted Average Remaining Contractual Term
Outstanding	7 years 8 months 12 days	6 years 8 months 12 days	6 years 7 months 6 days	5 years 1 month 6 days
Granted	6 years	5 years 10 months 24 days
Exercisable at end of the period	5 years 9 months 18 days	3 years 9 months 18 days
Aggregate Intrinsic Value
Outstanding	$ 0	$ 0	$ 0	$ 107,000
Exercisable at end of the period	$ 0	$ 0