Stock-Based Compensation - Equity Incentive Plans (Details) - shares	1 Months Ended	9 Months Ended
	Jun. 30, 2019	Sep. 30, 2021	Dec. 31, 2020
Class of Stock
Common stock options outstanding		5,328,971	4,614,323
Options outstanding under earlier plans		207,908
Grant of inducement stock option		325,000
2013 Stock Incentive Plan [Member]
Class of Stock
Common stock, shares reserved for future issuance	5,653,057
Common shares available for grant		93,469
Maximum number of additional common shares	868,372
Common stock options outstanding		4,796,063
Restricted stock units outstanding		576,371