SELECTED QUARTERLY FINANCIAL DATA (UNADUITED) - Schedule of Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 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, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues	$ 59,225	$ 55,147	$ 57,203	$ 57,929	$ 42,599	$ 77,493	$ 63,274	$ 128,404	$ 229,504	$ 311,770	$ 380,724
Income (loss) from operations	(190,985)	(10,133)	3,618	3,072	(13,082)	9,628	(4,225)	51,338	(194,428)	43,659	(42,180)
Net (loss) income	$ (192,626)	$ 3,331	$ (13,605)	$ (14,301)	$ (24,138)	$ 48,270	$ (21,048)	$ 33,824	$ (217,201)	$ 36,908	$ (102,496)
Basic net (loss) income per share (in dollars per share)	$ (2.65)	$ 0.05	$ (0.21)	$ (0.22)	$ (0.38)	$ 0.76	$ (0.33)	$ 0.53	$ (3.07)	$ 0.58	$ (1.63)
Diluted net (loss) income per share (in dollars per share)	$ (2.65)	$ 0.05	$ (0.21)	$ (0.22)	$ (0.38)	$ 0.65	$ (0.33)	$ 0.48	$ (3.07)	$ 0.57	$ (1.63)
Loss on impairment of intangible asset									$ 189,790	$ 0	$ 0
Product sales, net
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues	$ 28,917	$ 27,502	$ 25,937	$ 26,450	$ 29,339	$ 29,435	$ 26,838	$ 44,354	108,806	$ 129,966	$ 379,880
NUCYNTA | Product rights
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Loss on impairment of intangible asset									$ 189,790