STRATEGIC ALLIANCE AND COLLABORATION WITH SERVIER - Collaboration Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenue	$ 880	$ 695	$ 2,514	$ 372	$ 476	$ 944	$ 2,182	$ 4,784	$ 4,461	$ 8,386
Milestone payments
Disaggregation of Revenue [Line Items]
Total revenue									0	3,690
Research and development reimbursable costs
Disaggregation of Revenue [Line Items]
Total revenue									4,308	3,683
Collaboration revenue
Disaggregation of Revenue [Line Items]
Total revenue									$ 4,308	$ 7,373