FAIR VALUE MEASUREMENTS - Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Jun. 30, 2016
Assets:
Cash and cash equivalents	$ 200,481	$ 62,436	$ 65,188	$ 102,873
Debt securities, available-for-sale	255,786	336,088
Total	456,267	398,524
Liabilities:
Business combination-related contingent consideration	71,300	70,900			$ 25,000
Total	71,300	70,900
Quoted prices in active markets (Level 1)
Assets:
Cash and cash equivalents	190,483	62,436
Debt securities, available-for-sale	0	0
Total	190,483	62,436
Liabilities:
Business combination-related contingent consideration	0	0
Total	0	0
Significant other observable inputs (Level 2)
Assets:
Cash and cash equivalents	9,998	0
Debt securities, available-for-sale	255,786	336,088
Total	265,784	336,088
Liabilities:
Business combination-related contingent consideration	0	0
Total	0	0
Significant unobservable inputs (Level 3)
Assets:
Cash and cash equivalents	0	0
Debt securities, available-for-sale	0	0
Total	0	0
Liabilities:
Business combination-related contingent consideration	71,300	70,900
Total	$ 71,300	$ 70,900