DEBT - Narrative (Details) $ / shares in Units, shares in Thousands				1 Months Ended				3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 13, 2019 USD ($) trading_day $ / shares shares	Apr. 02, 2015 USD ($)	Sep. 09, 2014 USD ($)	Oct. 31, 2019 USD ($)	Jan. 31, 2019 USD ($)	Nov. 30, 2017 USD ($)	Apr. 30, 2017 USD ($)	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) $ / shares	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2015 USD ($)	Aug. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Debt issuance costs										$ 4,268,000	$ 0
Payment of debt extinguishment										30,000,000	0
Gain (loss) on extinguishment of debt								$ 26,385,000	$ 0	26,385,000	0
Accretion of debt discount and debt issuance costs										18,090,000	$ 16,298,000
Common Stock
Debt Instrument [Line Items]
Extinguishment of debt, shares issued (in shares) | shares	15,800							15,817
Common Stock | Beneficial Owner
Debt Instrument [Line Items]
Extinguishment of debt, shares issued (in shares) | shares	1,700
2.50% Convertible Notes due 2021
Debt Instrument [Line Items]
Gain (loss) on extinguishment of debt								$ 26,400,000
Reacquisition of equity component of 2021 Notes, net of tax								6,200,000
Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount		$ 575,000,000						$ 182,500,000		$ 182,500,000
Proceeds from the issuance of debt		$ 562,000,000
Debt issuance costs													$ 500,000
Effective interest rate								12.08%	12.15%	12.08%	12.15%
Debt extinguishment, amount						$ 10,000,000	$ 100,000,000
Payment of debt extinguishment						$ 400,000	$ 4,000,000					$ 4,400,000
Gain (loss) on extinguishment of debt												(5,900,000)
Unamortized balances of debt discount and debt issuance costs												$ 1,500,000
Amendment fee					$ 3,200,000
Principal payments, scheduled prior to September 30, 2020								$ 80,000,000		$ 80,000,000
Accretion of debt discount and debt issuance costs								1,534,000	$ 886,000	4,001,000	$ 2,747,000
Debt discount								5,220,000		5,220,000					$ 2,541,000
Senior Notes | Subsequent Event
Debt Instrument [Line Items]
Principal payment paid				$ 20,000,000
Senior Notes | Period One
Debt Instrument [Line Items]
Prepayment premium					3.00%
Senior Notes | Period Two
Debt Instrument [Line Items]
Prepayment premium					1.00%
Senior Notes | Three month LIBOR
Debt Instrument [Line Items]
Basis spread		9.75%
Interest rate through third anniversary of purchase date		11.95%
Interest rate after third anniversary of purchase date		12.95%
Senior Notes | Three month LIBOR | Minimum
Debt Instrument [Line Items]
Interest rate		1.00%
Senior Notes | NUCYNTA
Debt Instrument [Line Items]
Proceeds from the issuance of debt		$ 550,000,000
Convertible debt | 2.50% Convertible Notes due 2021
Debt Instrument [Line Items]
Aggregate principal amount	$ 145,000,000.0		$ 345,000,000
Interest rate			2.50%											2.50%
Debt extinguishment, amount	200,000,000
Unamortized balances of debt discount and debt issuance costs	18,900,000
Net proceeds from debt offering			$ 334,200,000
Underwriting discount			10,400,000
Offering expenses			$ 400,000
Extinguishment of debt, cash paid	30,000,000
Reacquisition of equity component of 2021 Notes, net of tax								(4,796,000)
Accretion of debt discount and debt issuance costs	26,100,000							$ 3,499,000	$ 4,604,000	$ 13,252,000	$ 13,551,000
Reduction in carrying amount of equity component	6,200,000
Carrying amount of equity component	112,800,000
Stock price trigger								130.00%		130.00%
Conversion price (in dollars per share) | $ / shares								$ 19.24		$ 19.24
Debt discount								$ 17,016,000		$ 17,016,000					$ 54,521,000
Convertible debt | 2.50% Convertible Notes due 2021 | Beneficial Owner
Debt Instrument [Line Items]
Aggregate principal amount	22,000,000
Payment of debt extinguishment	3,500,000
Convertible debt | 5.00% Convertible Notes due 2024
Debt Instrument [Line Items]
Aggregate principal amount	$ 120,000,000
Interest rate	5.00%
Effective interest rate	17.82%
Debt instrument, maximum borrowing capacity in connection with debt exchange (up to)														$ 30,000,000
Debt instrument, exit fee due, if circumstances met														$ 4,400,000
Reacquisition of equity component of 2021 Notes, net of tax								24,165,000
Accretion of debt discount and debt issuance costs								837,000		837,000
Carrying amount of equity component	$ 32,600,000
Stock price trigger	150.00%
Conversion price (in dollars per share) | $ / shares	$ 3.09
Principal amount, denominator for conversion	$ 1,000
Consecutive trading day observation period			40 days
Conversion ratio	0.3235198
Fundamental change occurred, principal amount, cash repurchase requirement	$ 1,000
Fundamental change occurred, principal amount multiple, cash repurchase requirement	$ 1,000
Fundamental change occurred, principal amount percentage, cash repurchase requirement	100.00%
Threshold trading days, whether or not consecutive | trading_day	20
Consecutive trading days | trading_day	30
Redemption price, percentage	100.00%
Debt default, percent of principal and unpaid interest due and payable immediately	100.00%
Fair value	$ 98,400,000
Carrying value of the liability component	65,800,000
Debt discount	54,200,000							$ 53,407,000		$ 53,407,000
Fair value discount	21,600,000
Total issuance costs	4,300,000
Debt issuance costs	2,900,000
Convertible debt | 5.00% Convertible Notes due 2024 | Beneficial Owner
Debt Instrument [Line Items]
Aggregate principal amount	$ 13,200,000