DEBT - Schedule of Principal Amounts of Senior Notes Repayable (Details) - Senior Notes - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
2018 (remainder)	$ 25,000
2019	120,000
2020	80,000
2021	82,500
Total	$ 307,500	$ 365,000