CASH, CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS - LEVEL 3 (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Sep. 09, 2014
Fair value transfers between hierarchy levels 1 and 2
Transfers of assets from Level 1 to Level 2	$ 0	$ 0	$ 0	$ 0
Transfer of assets from Level 2 to Level 1	0	0	0	0
Fair value transfers in or out of hierarchy level 3
Net transfer of liabilities into/out of level 3	$ 0	0	$ 0	0
Convertible debt
Estimated fair value of debt instruments
Interest rate (as a percent)	2.50%		2.50%		2.50%	2.50%
Par value of debt	$ 345,000		$ 345,000		$ 345,000	$ 345,000
Level 2 | Convertible debt | Fair Value
Estimated fair value of debt instruments
Estimated fair value of debt	281,200		281,200		$ 295,400
Contingent consideration | Level 3
Changes in fair value of all financial liabilities measured at fair value on a recurring basis
Fair value, beginning of the period	1,249	8,611	1,613	14,825
Total	967	7,356	967	7,356
Contingent consideration | Level 3 | Royalties
Changes in fair value of all financial liabilities measured at fair value on a recurring basis
Net accretion value adjustments	22	392	184	2,137
Contingent consideration | Level 3 | Interest Expense.
Changes in fair value of all financial liabilities measured at fair value on a recurring basis
Net accretion value adjustments	39	265	79	796
Contingent consideration | Level 3 | Selling general and administrative expense
Changes in fair value of all financial liabilities measured at fair value on a recurring basis
Net accretion value adjustments	$ (299)	$ (1,128)	$ (541)	$ (6,128)