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RELATED PARTY TRANSACTIONS - Narrative (Details)	1 Months Ended						6 Months Ended
RELATED PARTY TRANSACTIONS - Narrative (Details)	Jan. 31, 2025 USD ($) plan	Nov. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Mar. 31, 2022 USD ($)	Feb. 28, 2022 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2025 USD ($) shares
MSC Income
Related Party Transaction [Line Items]
Share repurchase program, authorized, amount	$ 65,000,000
MSC Income
Related Party Transaction [Line Items]
Share purchase program, authorized, amount	$ 20,000,000
Share purchase program, period in force	12 months
Share purchase (repurchase) program, number of active plans \| plan	2
Share purchase program, percentage of outstanding stock maximum (as a percent)	3.00%
Related Party \| 2015 Deferred Compensation Plan
Related Party Transaction [Line Items]
Amounts of transaction							$ 30,100,000
Related Party \| 2015 Deferred Compensation Plan \| Rabbi Trust
Related Party Transaction [Line Items]
Amounts of transaction							$ 18,700,000
Related Party \| 2015 Deferred Compensation Plan \| Common Stock
Related Party Transaction [Line Items]
Deferred compensation (in shares) \| shares							193,549
Related Party \| 2015 Deferred Compensation Plan \| Phantom Stock Units
Related Party Transaction [Line Items]
Amounts of transaction							$ 11,400,000
Related Party \| External Investment Manager
Related Party Transaction [Line Items]
Receivable							10,700,000
Related Party \| External Investment Manager \| Operating Expenses and Amount Due From Tax Sharing Agreement
Related Party Transaction [Line Items]
Receivable							7,800,000
Related Party \| External Investment Manager \| Dividends Declared But Not Yet Paid
Related Party Transaction [Line Items]
Receivable							$ 2,800,000
Related Party \| MSC Income
Related Party Transaction [Line Items]
Shares acquired (in shares) \| shares							1,377,952
Related Party \| Private Loan Fund I \| Investment Management Agreement
Related Party Transaction [Line Items]
Amounts of transaction					$ 15,000,000.0	$ 10,000,000.0
Funded commitments							$ 14,200,000
Unfunded commitments							800,000
Related Party \| Private Loan Fund I \| Secured Revolving Promissory Note
Related Party Transaction [Line Items]
Amounts of transaction				$ 15,000,000.0
Interest rate (as a percent)				3.00%
Interest rate, floor (as a percent)				2.00%
Unused capacity, commitment fee (as a percent)				0.25%
Amount of debt outstanding							4,000,000.0
Related Party \| Private Loan Fund I \| 2015 Deferred Compensation Plan \| Rabbi Trust
Related Party Transaction [Line Items]
Amounts of transaction							2,100,000
Related Party \| Private Loan Fund II \| Investment Management Agreement
Related Party Transaction [Line Items]
Amounts of transaction			$ 15,000,000.0
Funded commitments							9,000,000.0
Unfunded commitments							6,000,000.0
Percentage of total commitments (as a percent)			20.00%
Related Party \| Private Loan Fund II \| Secured Revolving Promissory Note
Related Party Transaction [Line Items]
Amounts of transaction		$ 10,000,000.0	$ 50,000,000.0
Interest rate (as a percent)		3.00%	3.50%
Interest rate, floor (as a percent)		2.00%	2.00%
Unused capacity, commitment fee (as a percent)		0.25%	0.25%
Amount of debt outstanding							0
Related Party \| Private Loan Fund II \| 2015 Deferred Compensation Plan \| Rabbi Trust
Related Party Transaction [Line Items]
Amounts of transaction							$ 4,500,000