INCOME TAXES - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended													12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Contingency [Line Items]
Dividends	$ 28,400
Dividends declared (in usd per share)	$ 0.63
Dividends cash paid (in usd per share)		$ 0.63	$ 0.62	$ 0.59	$ 0.58	$ 0.57	$ 0.50	$ 0.63	$ 0.48	$ 0.97	$ 0.54	$ 0.53	$ 0.52	$ 2.42	$ 2.18	$ 2.56
Undistributed ordinary income - tax basis	$ 28,845				$ 16,070				$ 12,682
Cumulative undistributed taxable income (in usd per share)	$ 0.64
Cost of investments	$ 1,471,100				1,210,800
Gross unrealized appreciation	96,300				72,300
Gross unrealized depreciation	96,400				76,800
Net unrealized appreciation (depreciation)	(100)				(4,500)
Deferred tax liability	11,997				$ 12,117
RIC
Income Tax Contingency [Line Items]
Cost of investments	1,425,900
Gross unrealized appreciation	16,700
Gross unrealized depreciation	85,200
Net unrealized appreciation (depreciation)	(68,500)
Taxable Subsidiary
Income Tax Contingency [Line Items]
Cost of investments	45,200
Gross unrealized appreciation	79,600
Gross unrealized depreciation	11,200
Net unrealized appreciation (depreciation)	$ 68,400
Regular Dividends
Income Tax Contingency [Line Items]
Dividends declared (in usd per share)	$ 0.57
Dividends cash paid (in usd per share)								$ 0.48		0.47
Supplemental Dividends
Income Tax Contingency [Line Items]
Dividends declared (in usd per share)	$ 0.06
Dividends cash paid (in usd per share)		$ 0.06	$ 0.06	$ 0.05	$ 0.05	$ 0.05				$ 0.50	$ 0.10	$ 0.10	$ 0.10
Interest-related Dividends
Income Tax Contingency [Line Items]
Percent of ordinary distributions														94.17%	89.74%
Non-U.S. residents and foreign corporation shareholders
Income Tax Contingency [Line Items]
Percent of ordinary distributions														94.17%	89.74%