| Consolidated Balance Sheets (Parenthetical) - USD ($) | Dec. 31, 2022 | Dec. 31, 2021 | ||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Investment, cost | $ 3,773,752,000 | [1],[2] | $ 3,259,246,000 | [3],[4] | ||||||||||||||||||||
| Deferred financing costs, accumulated amortization | $ 10,603,000 | $ 9,462,000 | ||||||||||||||||||||||
| Common stock, par value (in dollars per share) | $ 0.01 | $ 0.01 | ||||||||||||||||||||||
| Common stock, shares authorized (in shares) | 150,000,000 | 150,000,000 | ||||||||||||||||||||||
| Common stock, shares issued (in shares) | 78,463,599 | 70,700,885 | ||||||||||||||||||||||
| Common stock, shares outstanding (in shares) | 78,463,599 | 70,700,885 | ||||||||||||||||||||||
| Unsecured Notes | July 2026 Notes | ||||||||||||||||||||||||
| Debt instrument, par value | $ 500,000,000 | $ 500,000,000 | ||||||||||||||||||||||
| Unsecured Notes | May 2024 Notes | ||||||||||||||||||||||||
| Debt instrument, par value | 450,000,000 | 450,000,000 | ||||||||||||||||||||||
| Unsecured Notes | SBIC debentures | ||||||||||||||||||||||||
| Debt instrument, par value | 350,000,000 | 350,000,000 | ||||||||||||||||||||||
| Debt, due within one year | 16,000,000 | |||||||||||||||||||||||
| Unsecured Notes | December 2025 Notes | ||||||||||||||||||||||||
| Debt instrument, par value | 100,000,000 | |||||||||||||||||||||||
| Unsecured Notes | December 2022 Notes | ||||||||||||||||||||||||
| Debt instrument, par value | 185,000,000 | |||||||||||||||||||||||
| Control investments | ||||||||||||||||||||||||
| Investment, cost | 1,270,802,000 | [2],[5] | 1,107,597,000 | [4],[6] | ||||||||||||||||||||
| Affiliate investments | ||||||||||||||||||||||||
| Investment, cost | 635,536,000 | [2],[7] | 578,539,000 | [4],[8] | ||||||||||||||||||||
| Non‑Control/Non‑Affiliate investments | ||||||||||||||||||||||||
| Investment, cost | $ 1,867,414,000 | [2],[9] | $ 1,573,110,000 | [4],[10] | ||||||||||||||||||||
| 
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