CORPORATE DEBT - Aggregate Positions in Capital Structure of Borrowers (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt, total	$ 1,256,276	$ 1,011,275	[1]
First lien loans
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt, total	753,373	582,724
Second lien loans
Schedule of Held-to-maturity Securities [Line Items]
Corporate debt, total	$ 502,903	$ 428,551

[1] Derived from the audited consolidated financial statements at December 31, 2017.