Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	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, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2017
Quarterly Financial Information (Unaudited)
Net sales	$ 133,841	$ 172,794	$ 193,718	$ 155,054	$ 170,911	$ 174,386	$ 184,305	$ 154,961	$ 139,118	$ 165,720	$ 170,944	$ 161,559	$ 655,407	$ 684,563	$ 637,341
Settlement agreement										(4,000)					(4,000)
Total net sales									139,118	161,720	170,944	161,559	655,407	684,563	633,341
Cost of sales	84,499	107,477	123,908	95,913	104,383	107,329	96,855	87,290	80,240	89,290	82,891	79,707	379,601	363,729	300,030
Gross profit	49,342	65,317	69,810	59,141	66,528	67,057	87,450	67,671	58,878	72,430	88,053	81,852	243,610	288,706	301,213
Operating expenses	39,940	31,939	33,133	400,919	57,926	33,777	40,315	26,992	30,069	28,793	53,747	102,158	505,931	159,010	214,767
Operating income (loss)	9,402	33,378	36,677	(341,778)	8,602	33,280	47,135	40,679	28,809	43,637	34,306	(20,306)	(262,321)	129,696	86,446
Other loss	(19,532)	(21,374)	(21,668)	(21,350)	(20,844)	(22,785)	(14,975)	(19,999)	(19,983)	(21,371)	(22,578)	(21,964)	(2,018)	2,278	(244)
Income tax expense (benefit)	(2,544)	1,359	2,647	(75,600)	(883)	(2,275)	18,138	7,423	3,100	7,337	3,542	(12,882)	(74,138)	22,403	1,097
Less: Net income attributable to noncontrolling interest											14	20			34
Net income (loss) attributable to Lannett Company, Inc.	$ (7,586)	$ 10,645	$ 12,362	$ (287,528)	$ (11,359)	$ 12,770	$ 14,022	$ 13,257	$ 5,726	$ 14,929	$ 8,172	$ (29,408)	$ (272,107)	$ 28,690	$ (581)
Earnings (loss) per common share
Basic (in dollars per share)	$ (0.20)	$ 0.28	$ 0.33	$ (7.65)	$ (0.30)	$ 0.34	$ 0.38	$ 0.36	$ 0.16	$ 0.41	$ 0.22	$ (0.80)	$ (7.20)	$ 0.77	$ (0.02)
Diluted (in dollars per share)	$ (0.20)	$ 0.27	$ 0.32	$ (7.65)	$ (0.30)	$ 0.33	$ 0.37	$ 0.35	$ 0.15	$ 0.40	$ 0.22	$ (0.80)	$ (7.20)	$ 0.75	$ (0.02)