Net Loss Per Share - Basic and diluted net loss per share computation (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net loss attributable to CASI Pharmaceuticals, Inc.	$ (16,953)	$ (9,891)	$ (34,088)	$ (33,539)
Denominator:
Weighted average number of common shares	117,940	95,891	105,922	95,753
Denominator for basic and diluted net loss per share calculation	117,940	95,891	105,922	95,753
Net loss per share - Basic and diluted (in dollars per share)	$ (0.14)	$ (0.10)	$ (0.32)	$ (0.35)