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RELATED PARTY TRANSACTIONS (Details)	1 Months Ended					3 Months Ended	9 Months Ended
RELATED PARTY TRANSACTIONS (Details)	Nov. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Mar. 31, 2022 USD ($)	Feb. 28, 2022 USD ($)	Dec. 31, 2020 USD ($)	Mar. 31, 2026 USD ($)	Sep. 30, 2026 USD ($) shares	Aug. 31, 2026 USD ($) plan	Jan. 31, 2025
Related Party \| 2015 Deferred Compensation Plan
Related Party Transaction [Line Items]
Amounts of transaction							$ 35,700,000
Related Party \| 2015 Deferred Compensation Plan \| Rabbi Trust
Related Party Transaction [Line Items]
Amounts of transaction							$ 23,900,000
Related Party \| 2015 Deferred Compensation Plan \| Common Stock
Related Party Transaction [Line Items]
Deferred compensation (in shares) \| shares							213,818
Related Party \| 2015 Deferred Compensation Plan \| Phantom Stock Units
Related Party Transaction [Line Items]
Amounts of transaction							$ 11,800,000
External Investment Manager \| Related Party
Related Party Transaction [Line Items]
Accounts receivable from MSCC and its subsidiaries							11,500,000
External Investment Manager \| Related Party \| Operating Expenses and Amount Due From Tax Sharing Agreement
Related Party Transaction [Line Items]
Accounts receivable from MSCC and its subsidiaries							8,900,000
External Investment Manager \| Related Party \| Dividends Declared But Not Yet Paid
Related Party Transaction [Line Items]
Accounts receivable from MSCC and its subsidiaries							$ 2,600,000
MSC Income
Related Party Transaction [Line Items]
Share purchase program, authorized, amount								$ 20,000,000.0
Share repurchase program, authorized, amount								$ 20,000,000.0
Share purchase (repurchase) program, number of active plans \| plan								2
Share purchase program, percentage of outstanding stock maximum (as a percent)									3.00%
MSC Income \| Related Party
Related Party Transaction [Line Items]
Units (in shares) \| shares							2,078,997
MSC Income \| Related Party \| Unsecured Revolving Promissory Note
Related Party Transaction [Line Items]
Amounts of transaction						$ 30,000,000.0
Interest rate (as a percent)						4.50%
Interest rate, floor (as a percent)						2.00%
Unused capacity, commitment fee (as a percent)						0.25%
MSC Income \| Related Party \| Secured Revolving Promissory Note
Related Party Transaction [Line Items]
Amount of debt outstanding							$ 0
Private Loan Fund I \| Related Party \| Secured Revolving Promissory Note
Related Party Transaction [Line Items]
Amounts of transaction			$ 20,000,000.0
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							17,300,000
Private Loan Fund I \| Related Party \| Investment Management Agreement
Related Party Transaction [Line Items]
Amounts of transaction				$ 15,000,000.0	$ 10,000,000.0
Funded commitments							15,000,000.0
Private Loan Fund I \| Related Party \| 2015 Deferred Compensation Plan \| Rabbi Trust
Related Party Transaction [Line Items]
Amounts of transaction							2,200,000
Private Loan Fund II \| Related Party \| Secured Revolving Promissory Note
Related Party Transaction [Line Items]
Amounts of transaction	$ 10,000,000.0
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							0
Private Loan Fund II \| Related Party \| Investment Management Agreement
Related Party Transaction [Line Items]
Amounts of transaction		$ 15,000,000.0
Funded commitments							12,800,000
Percentage of total commitments (as a percent)		20.00%
Unfunded commitments							2,300,000
Private Loan Fund II \| Related Party \| 2015 Deferred Compensation Plan \| Rabbi Trust
Related Party Transaction [Line Items]
Amounts of transaction							$ 6,400,000