Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended												12 Months Ended
	Oct. 04, 2018	Jun. 30, 2020	Mar. 31, 2020	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, 2017	Sep. 30, 2017	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2018
Interim Financial Information
Net sales		$ 137,920	$ 144,372	$ 136,110	$ 127,342	$ 133,841	$ 172,794	$ 193,718	$ 155,054	$ 170,911	$ 174,386	$ 184,305	$ 154,961	$ 545,744	$ 655,407	$ 684,563
Cost of sales		98,328	102,696	94,816	84,684	84,499	107,477	123,908	95,913	104,383	107,329	96,855	87,290
Gross profit		39,592	41,676	41,294	42,658	49,342	65,317	69,810	59,141	66,528	67,057	87,450	67,671	165,220	243,610	288,706
Operating expenses		44,123	43,768	24,519	33,254	39,940	31,939	33,133	400,919	57,926	33,777	40,315	26,992	145,664	505,931	159,010
Operating income (loss)		(4,531)	(2,092)	16,775	9,404	9,402	33,378	36,677	(341,778)	8,602	33,280	47,135	40,679	19,556	(262,321)	129,696
Other loss		(15,247)	(16,164)	(16,999)	(19,774)	(19,532)	(21,374)	(21,668)	(21,350)	(20,844)	(22,785)	(14,975)	(19,999)	(68,184)	(83,924)	(78,603)
Income tax expense (benefit)		(10,077)	(1,664)	(5,308)	1,787	(2,544)	1,359	2,647	(75,600)	(883)	(2,275)	18,138	7,423	(15,262)	(74,138)	22,403
Net income (loss)		$ (9,701)	$ (16,592)	$ 5,084	$ (12,157)	$ (7,586)	$ 10,645	$ 12,362	$ (287,528)	$ (11,359)	$ 12,770	$ 14,022	$ 13,257	$ (33,366)	$ (272,107)	$ 28,690
Loss per common share attributable to Lannett Company, Inc.:
Basic (in dollars per share)		$ (0.25)	$ (0.43)	$ 0.13	$ (0.32)	$ (0.20)	$ 0.28	$ 0.33	$ (7.65)	$ (0.30)	$ 0.34	$ 0.38	$ 0.36	$ (0.86)	$ (7.20)	$ 0.77
Diluted (in dollars per share)		$ (0.25)	$ (0.43)	$ 0.13	$ (0.32)	$ (0.20)	$ 0.27	$ 0.32	$ (7.65)	$ (0.30)	$ 0.33	$ 0.37	$ 0.35	$ (0.86)	$ (7.20)	$ 0.75
Impairment of goodwill	$ 339,600								$ 339,600