Business, Liquidity and Capital Resources (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Business, Liquidity and Capital Resources (Details) [Line Items]
Cash and cash equivalents	$ 40,742	$ 165,905	$ 40,742	$ 165,905	$ 168,598
Net losses	(10,777)	(7,970)	(37,024)	(25,170)
Non-cash stock-based compensation charges	2,716	2,781	8,454	7,407
Liquidity, Capital Resources and Management Plans [Member]
Business, Liquidity and Capital Resources (Details) [Line Items]
Cash and cash equivalents	138,410		138,410		$ 168,598
Net losses	$ 10,777	$ 7,970	37,024	$ 25,170
Cash flows used in operating activities			27,595
Non-cash stock-based compensation charges			$ 8,454