Statements of Operations - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Statement [Abstract]
License and other revenue	$ 34	$ 4,097
Type of Revenue [Extensible List]	clsd:LicenseAndOtherRevenueMember	clsd:LicenseAndOtherRevenueMember
Operating expenses:
Research and development	$ 5,490	$ 3,811
General and administrative	2,893	3,122
Total operating expenses	8,383	6,933
Loss from operations	(8,349)	(2,836)
Other income	998	0
Other expense	0	(75)
Net loss	$ (7,351)	$ (2,911)
Net loss per share of common stock — basic and diluted	$ (0.13)	$ (0.07)
Weighted average shares outstanding — basic and diluted	57,038,664	44,753,510