COMMITMENTS AND CONTINGENCIES - Commitments to Fund (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Total commitments	$ 1,814,832	$ 1,847,534
Less: funded commitments	(1,668,324)	(1,737,286)
Total unfunded commitments	$ 146,508	$ 110,248