ORGANIZATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - SEGMENT INFORMATION (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) segment item	Dec. 31, 2016 USD ($)	Dec. 31, 2015
Segment Information
Number of operating segments | segment	1
Accounts receivable balances related to royalties | $	$ 0.5	$ 0.2
Supplier concentration risk | Gralise
Segment Information
Number of third-party suppliers for the supply of gabapentin	1
Product shipments | Customer concentration risk
Segment Information
Number of major distributors	3
Percentage of concentration risk	100.00%	100.00%	100.00%
Product shipments | Customer concentration risk | McKesson Corporation
Segment Information
Percentage of concentration risk	36.00%	36.00%	36.00%
Product shipments | Customer concentration risk | AmerisourceBergin Corporation
Segment Information
Percentage of concentration risk	27.00%	27.00%	24.00%
Product shipments | Customer concentration risk | Cardinal Health
Segment Information
Percentage of concentration risk	26.00%	25.00%	27.00%
Product shipments | Customer concentration risk | All others
Segment Information
Percentage of concentration risk	11.00%	12.00%	13.00%
Product sales related accounts receivable | Credit concentration risk
Segment Information
Percentage of concentration risk	100.00%	100.00%	100.00%
Product sales related accounts receivable | Credit concentration risk | McKesson Corporation
Segment Information
Percentage of concentration risk	41.00%	39.00%	38.00%
Product sales related accounts receivable | Credit concentration risk | AmerisourceBergin Corporation
Segment Information
Percentage of concentration risk	27.00%	33.00%	25.00%
Product sales related accounts receivable | Credit concentration risk | Cardinal Health
Segment Information
Percentage of concentration risk	23.00%	20.00%	25.00%
Product sales related accounts receivable | Credit concentration risk | All others
Segment Information
Percentage of concentration risk	9.00%	8.00%	12.00%