Fair Value - Level III Rollforward (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2023
Jun. 30, 2022
Jun. 30, 2023
Jun. 30, 2022
Total        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance $ 3,184,377 $ 3,046,993 $ 3,136,291 $ 2,843,456
Purchases, including capitalized PIK and revolver fundings 51,977 266,387 182,910 440,759
Proceeds from sales and paydowns of investments (141,689) (206,787) (232,298) (301,728)
Transfers into Level III 16,164 109,609 15,726 158,899
Transfers out of Level III (11,136) (75,436) (11,490) (10,000)
Ending balance 3,095,531 3,129,485 3,095,531 3,129,485
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: (11,932) (1,298) (3,100) 1,878
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 2,565 16,518 3,273 35,690
Total | Net change in unrealized appreciation (depreciation)        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gains or losses included in earnings (6,727) (27,799) 1,119 (37,591)
First lien        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance 1,798,746 1,711,500 1,753,967 1,635,143
Purchases, including capitalized PIK and revolver fundings 40,201 243,696 158,106 399,092
Proceeds from sales and paydowns of investments (131,359) (150,322) (202,592) (203,478)
Transfers into Level III 0 27,749 0 0
Transfers out of Level III (11,136) 0 (11,490) 0
Ending balance 1,693,215 1,835,954 1,693,215 1,835,954
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: (3,604) 3,991 7,374 3,775
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 (92) (595) (14,048) (664)
First lien | Net change in unrealized appreciation (depreciation)        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gains or losses included in earnings (3,145) 3,926 9,272 5,861
Second lien        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance 480,398 446,914 480,068 319,120
Purchases, including capitalized PIK and revolver fundings 997 2,495 1,942 11,846
Proceeds from sales and paydowns of investments (190) (6,722) (190) (14,500)
Transfers into Level III 16,164 81,860 15,726 158,899
Transfers out of Level III 0 (75,436) 0 (10,000)
Ending balance 495,695 434,069 495,695 434,069
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: 803 (15,042) 7,654 (31,296)
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 (7,223) 0 (11,934) 0
Second lien | Net change in unrealized appreciation (depreciation)        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gains or losses included in earnings 5,549 (15,042) 10,083 (31,296)
Subordinated        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance 74,348 54,867 72,842 50,742
Purchases, including capitalized PIK and revolver fundings 4,807 1,244 6,577 6,777
Proceeds from sales and paydowns of investments 0 0 0 0
Transfers into Level III 0 0 0 0
Transfers out of Level III 0 0 0 0
Ending balance 78,960 55,968 78,960 55,968
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: (195) (144) (352) (1,551)
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 appreciation (depreciation)        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gains or losses included in earnings (195) (143) (459) (1,551)
Equity and other        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance 830,885 833,712 829,414 838,451
Purchases, including capitalized PIK and revolver fundings 5,972 18,952 16,285 23,044
Proceeds from sales and paydowns of investments (10,140) (49,743) (29,516) (83,750)
Transfers into Level III 0 0 0 0
Transfers out of Level III 0 0 0 0
Ending balance 827,661 803,494 827,661 803,494
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: (8,936) 9,897 (17,776) 30,950
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 9,880 17,113 29,255 36,354
Equity and other | Net change in unrealized appreciation (depreciation)        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Total gains or losses included in earnings $ (8,936) $ (16,540) $ (17,777) $ (10,605)