FAIR VALUE MEASUREMENTS (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
FAIR VALUE MEASUREMENTS
Investments	$ 764,554	$ 819,208
First lien secured loans
FAIR VALUE MEASUREMENTS
Investments	615,945	697,232
Second lien secured loans
FAIR VALUE MEASUREMENTS
Investments	20,383	23,650
Subordinated unsecured loans
FAIR VALUE MEASUREMENTS
Investments	167
Subordinated Note to STRS JV
FAIR VALUE MEASUREMENTS
Investments	80,000	167
Subordinated Note to STRS JV
FAIR VALUE MEASUREMENTS
Investments		60,000
Equity (excluding STRS JV)
FAIR VALUE MEASUREMENTS
Investments	25,993	22,552
Equity in STRS JV
FAIR VALUE MEASUREMENTS
Investments	22,066	15,607
Level 2 | Foreign currency forward contracts
FAIR VALUE MEASUREMENTS
Investments	10
Level 3
FAIR VALUE MEASUREMENTS
Investments	742,488	803,601
Level 3 | First lien secured loans
FAIR VALUE MEASUREMENTS
Investments	615,945	697,232
Level 3 | Second lien secured loans
FAIR VALUE MEASUREMENTS
Investments	20,383	23,650
Level 3 | Subordinated unsecured loans
FAIR VALUE MEASUREMENTS
Investments	167
Level 3 | Subordinated Note to STRS JV
FAIR VALUE MEASUREMENTS
Investments	80,000	167
Level 3 | Subordinated Note to STRS JV
FAIR VALUE MEASUREMENTS
Investments		60,000
Level 3 | Equity (excluding STRS JV)
FAIR VALUE MEASUREMENTS
Investments	$ 25,993	$ 22,552