BUSINESS DEVELOPMENT - Asset Acquisition, Narrative (Details)			3 Months Ended		12 Months Ended
	Sep. 09, 2019 USD ($) asset	Jan. 04, 2019 USD ($)	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Schedule Of Asset Acquisition [Line Items]
Research and development			$ 20,816,000	$ 18,321,000
Rebound
Schedule Of Asset Acquisition [Line Items]
Upfront payment	$ 67,100,000
Contingent development milestone payments commitment (up to)	$ 35,000,000.0
Number of assets acquired | asset	1
Remaining net assets acquired	$ 7,200,000
Development milestone					$ 5,000,000.0
In-process research and development					$ 5,000,000.0
Research and development				$ 1,700,000
Rebound | Cash and Cash Equivalents
Schedule Of Asset Acquisition [Line Items]
Remaining net assets acquired	2,100,000
Rebound | Deferred Tax Asset
Schedule Of Asset Acquisition [Line Items]
Remaining net assets acquired	4,200,000
Rebound | In-Process Research and Development Expense
Schedule Of Asset Acquisition [Line Items]
Upfront payment	$ 59,900,000
ISC | Enabling technology license
Schedule Of Asset Acquisition [Line Items]
Upfront payment, intangible assets		$ 1,700,000
Option to acquire, period subsequent to first commercial sale		4 years
Maximum future purchase price, if circumstances met (not to exceed)		$ 80,000,000.0