Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	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, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2016
Quarterly Financial Information (Unaudited)
Net sales	$ 170,911	$ 174,386	$ 184,305	$ 154,961	$ 139,118	$ 165,720	$ 170,944	$ 161,559	$ 168,887	$ 163,712	$ 127,059	$ 106,433	$ 684,563	$ 637,341	$ 566,091
Settlement agreement						(4,000)				(23,598)				(4,000)	(23,598)
Total net sales					139,118	161,720	170,944	161,559	168,887	140,114	127,059	106,433	684,563	633,341	542,493
Cost of sales	104,383	107,329	96,855	87,290	80,240	89,290	82,891	79,707	88,957	82,623	55,414	29,006	363,729	300,030	237,371
Amortization of intangible assets													32,128	32,098	18,629
Gross profit	66,528	67,057	87,450	67,671	58,878	72,430	88,053	81,852	79,930	57,491	71,645	77,427	288,706	301,213	286,493
Operating expenses	57,926	33,777	40,315	26,992	30,069	28,793	53,747	102,158	49,535	38,874	41,320	26,006	159,010	214,767	155,735
Operating income	8,602	33,280	47,135	40,679	28,809	43,637	34,306	(20,306)	30,395	18,617	30,325	51,421	129,696	86,446	130,758
Other	(20,844)	(22,785)	(14,975)	(19,999)	(19,983)	(21,371)	(22,578)	(21,964)	(29,752)	(26,830)	(10,827)	(1,170)	2,278	(244)	(1)
Income tax expense	(883)	(2,275)	18,138	7,423	3,100	7,337	3,542	(12,882)	(2,948)	(2,743)	5,958	17,055	22,403	1,097	17,322
Less: Net income attributable to noncontrolling interest							14	20	20	20	20	15		34	75
Net income (loss) attributable to Lannett Company, Inc.	$ (11,359)	$ 12,770	$ 14,022	$ 13,257	$ 5,726	$ 14,929	$ 8,172	$ (29,408)	$ 3,571	$ (5,490)	$ 13,520	$ 33,181	$ 28,690	$ (581)	$ 44,782
Earnings (loss) per common share attributable to Lannett Company, Inc.:
Basic (in dollars per share)	$ (0.30)	$ 0.34	$ 0.38	$ 0.36	$ 0.16	$ 0.41	$ 0.22	$ (0.80)	$ 0.10	$ (0.15)	$ 0.37	$ 0.91	$ 0.77	$ (0.02)	$ 1.23
Diluted (in dollars per share)	$ (0.30)	$ 0.33	$ 0.37	$ 0.35	$ 0.15	$ 0.40	$ 0.22	$ (0.80)	$ 0.10	$ (0.15)	$ 0.36	$ 0.89	$ 0.75	$ (0.02)	$ 1.20