Annual report [Section 13 and 15(d), not S-K Item 405]

DEBT - Notes??Due and SBIC Debentures (Details)

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DEBT - Notes Due and SBIC Debentures (Details)
1 Months Ended 12 Months Ended
Sep. 30, 2024
USD ($)
Jun. 30, 2024
USD ($)
May 31, 2024
USD ($)
Mar. 31, 2024
USD ($)
Jan. 31, 2024
USD ($)
Dec. 31, 2022
USD ($)
agency
Oct. 31, 2021
USD ($)
Jan. 31, 2021
USD ($)
Dec. 31, 2024
USD ($)
Dec. 31, 2023
USD ($)
Dec. 31, 2022
USD ($)
agency
Feb. 28, 2023
USD ($)
Debt Instrument [Line Items]                        
Debt instrument, payable                 $ 2,134,000,000 $ 1,810,000,000    
Debt                 2,122,371,000 1,802,344,000    
July 2026 Notes | Unsecured Notes                        
Debt Instrument [Line Items]                        
Debt instrument, face amount             $ 200,000,000.0 $ 300,000,000.0 500,000,000 500,000,000    
Debt instrument, interest rate, stated (as a percent)               3.00%        
Debt instrument, issuance price (as a percent)             101.741% 99.004%        
Debt instrument, payable                 500,000,000 500,000,000    
Debt                 499,188,000 498,662,000    
June 2027 Notes                        
Debt Instrument [Line Items]                        
Proceeds from debt                 400,000,000 0 $ 0  
June 2027 Notes | Unsecured Notes                        
Debt Instrument [Line Items]                        
Debt instrument, face amount $ 400,000,000.0 $ 300,000,000.0             400,000,000      
Debt instrument, interest rate, stated (as a percent) 6.34% 6.50%                    
Debt instrument, issuance price (as a percent) 102.134% 99.793%                    
Debt instrument, payable                 400,000,000      
Debt                 399,282,000 0    
June 2027 Notes, Additional | Unsecured Notes                        
Debt Instrument [Line Items]                        
Debt instrument, face amount $ 100,000,000.0                      
Debt instrument, interest rate, stated (as a percent) 5.617%                      
March 2029 Notes                        
Debt Instrument [Line Items]                        
Proceeds from debt                 350,000,000 0 0  
March 2029 Notes | Unsecured Notes                        
Debt Instrument [Line Items]                        
Debt instrument, face amount         $ 350,000,000.0       350,000,000      
Debt instrument, interest rate, stated (as a percent)         6.95%              
Debt instrument, issuance price (as a percent)         99.865%              
Debt instrument, payable                 350,000,000      
Debt                 347,002,000 0    
SBIC Debentures                        
Debt Instrument [Line Items]                        
Repayments of debt                 63,800,000 16,000,000 0  
Proceeds from debt                 63,800,000 16,000,000 0  
SBIC Debentures | Unsecured Notes                        
Debt Instrument [Line Items]                        
Debt instrument, face amount                 350,000,000 350,000,000    
Debt instrument, payable $ 350,000,000.0     $ 286,200,000         $ 350,000,000 $ 350,000,000    
Repayments of debt       $ 63,800,000                
Proceeds from debt $ 63,800,000                      
Debt instrument, term (in years)                 10 years      
Debt instrument, weighted-average annual interest rate (as a percent)                 3.30% 3.00%    
Debt instrument, remaining term (in years)                 5 years 7 months 6 days      
Debt                 $ 343,417,000 $ 344,535,000    
SBIC Debentures | Unsecured Notes | Maximum                        
Debt Instrument [Line Items]                        
Debt instrument, payable                 350,000,000.0      
SBIC Debentures | Unsecured Notes | MSMF                        
Debt Instrument [Line Items]                        
Debt instrument, face amount                 175,000,000.0      
Debt                 170,300,000      
Debt instrument, unamortized debt issuance costs                 4,700,000      
SBIC Debentures | Unsecured Notes | MSC III                        
Debt Instrument [Line Items]                        
Debt instrument, face amount                 175,000,000.0      
Debt                 173,100,000      
Debt instrument, unamortized debt issuance costs                 1,900,000      
December 2025 Notes                        
Debt Instrument [Line Items]                        
Proceeds from debt                 0 50,000,000 $ 100,000,000  
December 2025 Notes | Unsecured Notes                        
Debt Instrument [Line Items]                        
Debt instrument, face amount                 150,000,000 150,000,000    
Debt instrument, payable                 150,000,000 150,000,000    
Debt                 149,482,000 148,965,000    
Debt instrument, credit rating, below investment grade event, percentage of agencies (as a percent)           67.00%         67.00%  
Debt instrument, secured debt ratio event           0.35         0.35  
December 2025 Notes | Unsecured Notes | Minimum                        
Debt Instrument [Line Items]                        
Debt instrument, credit rating, below investment grade rating, number of agencies | agency           1         1  
December 2025 Notes | Unsecured Notes | Maximum                        
Debt Instrument [Line Items]                        
Debt instrument, credit rating, below investment grade rating, number of agencies | agency           2         2  
December 2025 Series A Notes | Unsecured Notes                        
Debt Instrument [Line Items]                        
Debt instrument, face amount           $ 100,000,000.0         $ 100,000,000.0  
Debt instrument, interest rate, stated (as a percent)           7.84%         7.84%  
December 2025 Series B Notes | Unsecured Notes                        
Debt Instrument [Line Items]                        
Debt instrument, face amount                       $ 50,000,000.0
Debt instrument, interest rate, stated (as a percent)                       7.53%
May 2024 Notes | Unsecured Notes                        
Debt Instrument [Line Items]                        
Debt instrument, face amount                   450,000,000.0    
Debt instrument, interest rate, stated (as a percent)     5.20%                  
Debt instrument, payable                   450,000,000    
Repayments of debt     $ 450,000,000.0                  
Debt                 $ 0 $ 450,182,000    
December 2022 Notes | Unsecured Notes                        
Debt Instrument [Line Items]                        
Debt instrument, interest rate, stated (as a percent)           4.50%         4.50%  
Repayments of debt           $ 185,000,000.0