REVENUE - Contract Assets and Liabilities (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended	48 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2021	Jan. 31, 2018	Dec. 31, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract asset	$ 32,051,000		$ 32,051,000					$ 0
Net revenues	77,493,000	$ 95,413,000	269,171,000	$ 286,317,000
Royalties and milestones
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Net revenues	20,277,000	$ 209,000	25,784,000	$ 596,000
Collegium Pharmaceutical Inc
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract liability							$ 10,000,000
Contract asset	32,051,000		32,051,000					$ 0
Grunenthal | Royalties and milestones | Commercialization agreement, net
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Increase in royalties obligation	10,800,000		25,200,000
Grunenthal | Maximum | Forecast
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty obligation						$ 9,000,000
Grunenthal | Royalties and milestones | Commercialization agreement, net
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Net revenues	7,100,000		21,500,000
NUCYNTA products | Grunenthal | Royalties and milestones | Commercialization agreement, net
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty revenue recognized	3,700,000
NUCYNTA products | Grunenthal | Minimum | Forecast
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Net revenues						180,000,000
NUCYNTA products | Grunenthal | Maximum | Forecast
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Net revenues						243,000,000
NUCYNTA products | Collegium Pharmaceutical Inc | Grunenthal | Minimum | Forecast
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty obligation					$ 34,000,000	$ 34,000,000
Prepaid and other current assets
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract asset	9,900,000		9,900,000
Other long-term assets
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract asset	$ 22,100,000		$ 22,100,000