FAIR VALUE MEASUREMENTS - Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2016
Assets:
Cash and cash equivalents	$ 237,170	$ 62,436	$ 75,657	$ 102,873
Debt securities, available-for-sale	220,206	336,088
Total	457,376	398,524
Liabilities:
Business combination-related contingent consideration	68,600	70,900			$ 25,000
Total	68,600	70,900
Quoted prices in active markets (Level 1)
Assets:
Cash and cash equivalents	237,170	62,436
Debt securities, available-for-sale	0	0
Total	237,170	62,436
Liabilities:
Business combination-related contingent consideration	0	0
Total	0	0
Significant other observable inputs (Level 2)
Assets:
Cash and cash equivalents	0	0
Debt securities, available-for-sale	220,206	336,088
Total	220,206	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	68,600	70,900
Total	$ 68,600	$ 70,900