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Fair Value Measurements - Fair Value Levels (Details) - USD ($) $ in Thousands | Sep. 30, 2023 | Dec. 31, 2022 | Dec. 31, 2021
--- | --- | --- | ---
Fair Value Measurements |  |  |
Fair Value | $ 3,123,450 | $ 2,873,588 | $ 2,387,374
Level 1 |  |  |
Fair Value Measurements |  |  |
Fair Value | 0 | 0 |
Level 2 |  |  |
Fair Value Measurements |  |  |
Fair Value | 68,585 | 30,821 | 17,064
Level 3 |  |  |
Fair Value Measurements |  |  |
Fair Value | 3,040,160 | 2,828,035 | 2,356,559
Level 1, Level 2, and Level 3 |  |  |
Fair Value Measurements |  |  |
Fair Value | 3,108,745 | 2,858,856 | 2,373,623
Net Asset Value |  |  |
Fair Value Measurements |  |  |
Fair Value | 14,705 | 14,732 | 13,751
First Lien Debt | Level 1 |  |  |
Fair Value Measurements |  |  |
Fair Value | 0 | 0 |
First Lien Debt | Level 2 |  |  |
Fair Value Measurements |  |  |
Fair Value | 25,881 | 25,362 | 17,064
First Lien Debt | Level 3 |  |  |
Fair Value Measurements |  |  |
Fair Value | 2,907,989 | 2,668,749 | 2,207,036
First Lien Debt | Level 1, Level 2, and Level 3 |  |  |
Fair Value Measurements |  |  |
Fair Value | 2,933,870 | 2,694,111 | 2,224,100
Second Lien Debt | Level 1 |  |  |
Fair Value Measurements |  |  |
Fair Value | 0 | 0 |
Second Lien Debt | Level 2 |  |  |
Fair Value Measurements |  |  |
Fair Value | 42,704 | 5,459 |
Second Lien Debt | Level 3 |  |  |
Fair Value Measurements |  |  |
Fair Value | 92,008 | 122,891 | 121,550
Second Lien Debt | Level 1, Level 2, and Level 3 |  |  |
Fair Value Measurements |  |  |
Fair Value | 134,712 | 128,350 | 121,550
Other Securities | Level 1 |  |  |
Fair Value Measurements |  |  |
Fair Value | 0 | 0 |
Other Securities | Level 2 |  |  |
Fair Value Measurements |  |  |
Fair Value | 0 | 0 |
Other Securities | Level 3 |  |  |
Fair Value Measurements |  |  |
Fair Value | 40,163 | 36,395 | 27,973
Other Securities | Level 1, Level 2, and Level 3 |  |  |
Fair Value Measurements |  |  |
Fair Value | $ 40,163 | $ 36,395 | $ 27,973

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