FAIR VALUE MEASUREMENTS - Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Liabilities:
Derivative liability related to warrants	$ 0	$ 15,710
Recurring basis
Assets:
Cash and cash equivalents	89,305	99,394
Marketable securities, available-for-sale	166,381	201,236
Total	255,686	300,630
Liabilities:
Derivative liability related to warrants		15,710
Business combination-related contingent consideration	91,500	90,000
Total	91,500	105,710
Recurring basis | Quoted prices in active markets (Level 1)
Assets:
Cash and cash equivalents	89,305	92,726
Marketable securities, available-for-sale	0	0
Total	89,305	92,726
Liabilities:
Derivative liability related to warrants		0
Business combination-related contingent consideration	0	0
Total	0	0
Recurring basis | Significant other observable inputs (Level 2)
Assets:
Cash and cash equivalents	0	6,668
Marketable securities, available-for-sale	166,381	201,236
Total	166,381	207,904
Liabilities:
Derivative liability related to warrants		0
Business combination-related contingent consideration	0	0
Total	0	0
Recurring basis | Significant unobservable inputs (Level 3)
Assets:
Cash and cash equivalents	0	0
Marketable securities, available-for-sale	0	0
Total	0	0
Liabilities:
Derivative liability related to warrants		15,710
Business combination-related contingent consideration	91,500	90,000
Total	$ 91,500	$ 105,710