Fair Value - Level III Rollforward (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Total        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance $ 3,095,531 $ 3,129,485 $ 3,136,291 $ 2,843,456
Purchases, including capitalized PIK and revolver fundings 33,311 147,022 216,221 587,780
Proceeds from sales and paydowns of investments (88,751) (176,517) (321,049) (478,245)
Transfers into Level III   79,611 15,726 230,968
Transfers out of Level III     (11,490)  
Ending balance 3,027,649 3,155,160 3,027,649 3,155,160
Unrealized (depreciation) appreciation for the period relating to those Level III assets that were still held by the Company at the end of the period (17,605) (24,455) (35,115) (23,102)
Total | Net realized gains (losses) on investments        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gains or losses included in earnings 4,671 62 7,944 35,753
Total | Net change in unrealized (depreciation) appreciation        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gains or losses included in earnings (17,113) (24,503) (15,994) (64,552)
First lien        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance 1,693,215 1,835,954 1,753,967 1,635,143
Purchases, including capitalized PIK and revolver fundings 26,051 116,907 184,157 515,997
Proceeds from sales and paydowns of investments (39,990) (175,045) (242,582) (378,522)
Transfers into Level III   0 0 0
Transfers out of Level III     (11,490)  
Ending balance 1,680,667 1,758,691 1,680,667 1,758,691
Unrealized (depreciation) appreciation for the period relating to those Level III assets that were still held by the Company at the end of the period 1,695 (19,123) (4,814) (13,414)
First lien | Net realized gains (losses) on investments        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gains or losses included in earnings 111 45 (1,244) (618)
First lien | Net change in unrealized (depreciation) appreciation        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gains or losses included in earnings 1,280 (19,170) (2,141) (13,309)
Second lien        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance 495,695 434,069 480,068 319,120
Purchases, including capitalized PIK and revolver fundings 1,244 7,422 3,185 19,269
Proceeds from sales and paydowns of investments (44,190) (1,448) (44,381) (15,949)
Transfers into Level III   79,611 15,726 230,968
Transfers out of Level III     0  
Ending balance 461,833 505,696 461,833 505,696
Unrealized (depreciation) appreciation for the period relating to those Level III assets that were still held by the Company at the end of the period 8,177 (13,958) 15,305 (47,712)
Second lien | Net realized gains (losses) on investments        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gains or losses included in earnings 0 0 (24,627) 0
Second lien | Net change in unrealized (depreciation) appreciation        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gains or losses included in earnings 9,084 (13,958) 31,862 (47,712)
Subordinated        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance 78,960 55,968 72,842 50,742
Purchases, including capitalized PIK and revolver fundings 1,408 18,321 7,986 25,099
Proceeds from sales and paydowns of investments 0 0 0 0
Transfers into Level III   0 0 0
Transfers out of Level III     0  
Ending balance 80,577 73,763 80,577 73,763
Unrealized (depreciation) appreciation for the period relating to those Level III assets that were still held by the Company at the end of the period 209 (526) (143) (2,078)
Subordinated | Net realized gains (losses) on investments        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gains or losses included in earnings 0 0 0 0
Subordinated | Net change in unrealized (depreciation) appreciation        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gains or losses included in earnings 209 (526) (251) (2,078)
Equity and other        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance 827,661 803,494 829,414 838,451
Purchases, including capitalized PIK and revolver fundings 4,608 4,372 20,893 27,415
Proceeds from sales and paydowns of investments (4,571) (24) (34,086) (83,774)
Transfers into Level III   0 0 0
Transfers out of Level III     0  
Ending balance 804,572 817,010 804,572 817,010
Unrealized (depreciation) appreciation for the period relating to those Level III assets that were still held by the Company at the end of the period (27,686) 9,152 (45,463) 40,102
Equity and other | Net realized gains (losses) on investments        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gains or losses included in earnings 4,560 17 33,815 36,371
Equity and other | Net change in unrealized (depreciation) appreciation        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gains or losses included in earnings $ (27,686) $ 9,151 $ (45,464) $ (1,453)