FAIR VALUE MEASUREMENT AND FINANCIAL INSTRUMENTS - Narrative (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Royalty rate	2.00%		2.00%
Debt Instrument, Index Rate, High-Yield	0.075		0.075		0.084
Change in fair value of contingent consideration	$ (114,000)	$ 18,400	$ (52,240)	$ (16,820)
Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Probability of achieving clinical and regulatory milestones	45.00%		45.00%
Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Probability of achieving clinical and regulatory milestones	55.00%		55.00%
Discount Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted-average cost of capital	860.00%		860.00%		8.80%
Discount Rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration liability measurements used	0.075		0.075		0.084
Discount Rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration liability measurements used	0.086		0.086		0.088