CAPITALIZED FEES PAID TO A RELATED PARTY (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Capitalized Fees paid to a Related Party
Net carrying value	$ 152,899	$ 166,722
Weighted average remaining amortization period	11 years 1 month 6 days
Capitalized Fees paid to a Related Party, Future amortization
Estimated amortization expense for the year 2019	$ 13,800
Estimated amortization expense for the year 2020	13,800
Estimated amortization expense for the year 2021	13,800
Estimated amortization expense for the year 2022	13,800
Estimated amortization expense for the year 2023	13,800
Estimated amortization expense thereafter	83,900
Royalty revenue from a related party
Capitalized Fees paid to a Related Party
Amortization expense	13,823	13,823	$ 13,823
GSK
Capitalized Fees paid to a Related Party
Gross carrying value	220,000	220,000
Accumulated amortization	(67,101)	(53,278)
Net carrying value	152,899	166,722
GSK | Royalty revenue from a related party
Capitalized Fees paid to a Related Party
Amortization expense	13,823	13,823	$ 13,823
GSK | United States
Capitalized Fees paid to a Related Party
Gross carrying value	120,000	120,000
GSK | Europe
Capitalized Fees paid to a Related Party
Gross carrying value	60,000	60,000
GSK | Japan
Capitalized Fees paid to a Related Party
Gross carrying value	$ 40,000	$ 40,000