Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]	Total
Balance at Dec. 31, 2018	$ 0	$ 954	$ 596,712	$ (8,034)	$ (1,227)	$ (478,941)	$ 109,464
Balance (in Shares) at Dec. 31, 2018	0	95,287,268
Issuance of common stock for options exercised	$ 0	$ 1	113	0	0	0	114
Issuance of common stock for options exercised (in shares)	0	64,137
Issuance of common stock for options and warrants exercised (in shares)	0
Repurchase of stock options to satisfy tax withholding obligations	$ 0	$ 0	(12)	0	0	0	(12)
Stock issuance costs	0	0	(8)	0	0	0	(8)
Stock-based compensation expense, net of forfeitures	0	0	5,320	0	0	0	5,320
Foreign currency translation adjustment	0	0	0	0	(2,636)	0	(2,636)
Issuance of common stock from exercise of warrants	0	$ 9	1,659	0	0	0	1,668
Issuance of common stock from exercise of warrants (in shares)		987,653
Net loss attributable to CASI Pharmaceuticals, Inc.	0	$ 0	(406)	0	0	(33,133)	(33,539)
Balance at Sep. 30, 2019	$ 0	$ 964	603,378	(8,034)	(3,863)	(512,074)	80,371
Balance (in shares) at Sep. 30, 2019	0	96,339,058
Balance at Jun. 30, 2019	$ 0	$ 958	600,569	(8,034)	(2,027)	(502,428)	89,038
Balance (in Shares) at Jun. 30, 2019	0	95,717,052
Issuance of common stock for options exercised	$ 0	$ 0	75	0	0	0	75
Issuance of common stock for options exercised (in shares)	0	45,875
Issuance of common stock from exercise of warrants	$ 0	$ 6	968	0	0	0	974
Issuance of common stock from exercise of warrants (in shares)	0	576,131
Stock-based compensation expense, net of forfeitures	$ 0	$ 0	2,011	0	0	0	2,011
Foreign currency translation adjustment	0	0	0	0	(1,836)	0	(1,836)
Net loss attributable to CASI Pharmaceuticals, Inc.	0	0	(245)	0	0	(9,646)	(9,891)
Balance at Sep. 30, 2019	$ 0	$ 964	603,378	(8,034)	(3,863)	(512,074)	80,371
Balance (in shares) at Sep. 30, 2019	0	96,339,058
Balance at Dec. 31, 2019	$ 0	$ 979	606,686	(8,034)	(2,728)	(523,908)	$ 72,995
Balance (in Shares) at Dec. 31, 2019	0	97,771,698
Issuance of common stock for options exercised (in shares)							2,789,473
Issuance of common stock for options and warrants exercised	$ 0	$ 27	3,847	0	0	0	$ 3,874
Issuance of common stock for options and warrants exercised (in shares)	0	2,737,795
Repurchase of stock options to satisfy tax withholding obligations	$ 0	$ 0	(251)	0	0	0	(251)
Issuance of common stock from exercise of warrants (in shares)	0
Issuance of common stock pursuant to financing agreements	$ 0	$ 234	44,865	0	0	0	45,099
Issuance of common stock pursuant to financing agreements (in shares)		23,434,336
Stock issuance costs	0	$ 0	(3,008)	0	0	0	(3,008)
Stock-based compensation expense, net of forfeitures	0	0	5,685	0	0	0	5,685
Foreign currency translation adjustment	0	0	0	0	1,221	0	1,221
Net loss attributable to CASI Pharmaceuticals, Inc.	0	0	(1,185)	0	0	(32,903)	(34,088)
Balance at Sep. 30, 2020	$ 0	$ 1,240	656,639	(8,034)	(1,507)	(556,811)	91,527
Balance (in shares) at Sep. 30, 2020	0	123,943,829
Balance at Jun. 30, 2020	$ 0	$ 1,010	614,617	(8,034)	(3,890)	(540,364)	63,339
Balance (in Shares) at Jun. 30, 2020	0	100,928,829
Issuance of common stock for options exercised	$ 0	$ 0	22	0	0	0	22
Issuance of common stock for options exercised (in shares)	0	15,000
Issuance of common stock pursuant to financing agreements	$ 0	$ 230	43,470	0	0	0	43,700
Issuance of common stock pursuant to financing agreements (in shares)		23,000,000
Stock issuance costs	0	$ 0	(2,757)	0	0	0	(2,757)
Stock-based compensation expense, net of forfeitures	0	0	1,793	0	0	0	1,793
Foreign currency translation adjustment	0	0	0	0	2,383	0	2,383
Net loss attributable to CASI Pharmaceuticals, Inc.	0	0	(506)	0	0	(16,447)	(16,953)
Balance at Sep. 30, 2020	$ 0	$ 1,240	$ 656,639	$ (8,034)	$ (1,507)	$ (556,811)	$ 91,527
Balance (in shares) at Sep. 30, 2020	0	123,943,829