Quarterly report pursuant to Section 13 or 15(d)

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

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DEBT - Notes Due and SBIC Debentures (Details)
1 Months Ended 3 Months Ended
Mar. 01, 2024
USD ($)
Jan. 31, 2024
USD ($)
Oct. 31, 2021
USD ($)
Jan. 31, 2021
USD ($)
Jul. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Apr. 30, 2019
USD ($)
Mar. 31, 2024
USD ($)
Mar. 31, 2023
USD ($)
Dec. 31, 2023
USD ($)
Feb. 28, 2023
USD ($)
Dec. 31, 2022
USD ($)
agency
Debt Instrument [Line Items]                        
Debt instrument, payable               $ 2,059,200,000   $ 1,810,000,000    
Debt               2,048,415,000   $ 1,802,344,000    
Unsecured Notes                        
Debt Instrument [Line Items]                        
Debt instrument, weighted-average annual interest rate (as a percent)                   3.00%    
July 2026 Notes | Unsecured Notes                        
Debt Instrument [Line Items]                        
Debt instrument, face amount     $ 200,000,000 $ 300,000,000       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               498,794,000   498,662,000    
May 2024 Notes | Unsecured Notes                        
Debt Instrument [Line Items]                        
Debt instrument, face amount         $ 125,000,000 $ 75,000,000 $ 250,000,000 450,000,000   450,000,000    
Debt instrument, interest rate, stated (as a percent)             5.20%          
Debt instrument, issuance price (as a percent)         102.70% 105.00% 99.125%          
Debt instrument, payable               450,000,000   450,000,000    
Debt               450,045,000   450,182,000    
March 2029 Notes | Unsecured Notes                        
Debt Instrument [Line Items]                        
Debt instrument, face amount   $ 350,000,000           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               346,469,000   0    
SBIC Debentures                        
Debt Instrument [Line Items]                        
Repayments of SBIC debentures               63,800,000 $ 16,000,000      
SBIC Debentures | Unsecured Notes                        
Debt Instrument [Line Items]                        
Debt instrument, face amount               286,200,000   350,000,000    
Debt instrument, payable               $ 286,200,000   350,000,000    
Debt instrument, term (in years)               10 years        
Repayments of SBIC debentures $ 63,800,000                      
Debt instrument, weighted-average annual interest rate (as a percent)               2.80%        
Debt instrument, remaining term (in years)               5 years 4 months 24 days        
Debt               $ 281,013,000   344,535,000    
SBIC Debentures | Unsecured Notes | Maximum                        
Debt Instrument [Line Items]                        
Debt instrument, payable               350,000,000        
SBIC Debentures | Unsecured Notes | MSMF                        
Debt Instrument [Line Items]                        
Debt instrument, face amount               111,200,000        
Debt               108,400,000        
Debt instrument, unamortized debt issuance costs               2,800,000        
SBIC Debentures | Unsecured Notes | MSC III                        
Debt Instrument [Line Items]                        
Debt instrument, face amount               175,000,000        
Debt               172,600,000        
Debt instrument, unamortized debt issuance costs               2,400,000        
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               $ 149,094,000   $ 148,965,000    
Debt instrument, credit rating, below investment grade event, percentage of agencies (as a percent)                   67.00%    
Debt instrument, secured debt ratio event                       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
December 2025 Notes | Unsecured Notes | Maximum                        
Debt Instrument [Line Items]                        
Debt instrument, credit rating, below investment grade rating, number of agencies | agency                       2
December 2025 Series A Notes | Unsecured Notes                        
Debt Instrument [Line Items]                        
Debt instrument, face amount                       $ 100,000,000
Debt instrument, interest rate, stated (as a percent)                       7.84%
December 2025 Series B Notes | Unsecured Notes                        
Debt Instrument [Line Items]                        
Debt instrument, face amount                     $ 50,000,000  
Debt instrument, interest rate, stated (as a percent)                     7.53%