Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								6 Months Ended	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2019	Jun. 30, 2018	Jun. 30, 2017
Consolidated Statements of Comprehensive Loss Data:
Revenues	$ 295	$ 0	$ 0	$ 0	$ 0	$ 0	$ 387	$ 482	$ 0	$ 295	$ 2,156	$ 3,091
Net loss attributable to Immunomedics, Inc. stockholders	$ (99,608)	$ (94,292)	$ (75,953)	$ (87,337)	$ (93,499)	$ (64,169)	$ (117,032)	$ (35,546)	$ (157,668)	$ (357,190)	$ (273,837)	$ (153,206)
Loss per common share attributable to Immunomedics, Inc. stockholders (basic and diluted) (in USD per share)	$ (0.50)	$ (0.49)	$ (0.40)	$ (0.46)	$ (0.50)	$ (0.34)	$ (0.68)	$ (0.21)	$ (0.84)	$ (1.84)	$ (1.78)	$ (1.47)
Weighted average shares used to calculate loss per common share (basic and diluted) (in shares)	199,614	191,981	191,745	191,052	190,171	186,937	171,124	166,054	188,554	193,617	153,475	104,536