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Investments -Schedule of Net Investment Income (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2024 | Sep. 30, 2023
Net Investment Income [Line Items] |  |  |  |
Net investment income earned | $ 117,759 | $ 100,863 | $ 334,250 | $ 290,065
Fixed income securities |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Net investment income earned | 98,464 | 90,013 | 286,501 | 254,016
Commercial mortgage loans ("CMLs") |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Net investment income earned | 3,238 | 2,516 | 9,177 | 6,680
Equity securities |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Net investment income earned | 5,362 | 2,083 | 12,147 | 5,524
Short-term investments |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Net investment income earned | 6,457 | 3,941 | 14,656 | 11,483
Alternative investments |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Net investment income earned | 9,031 | 6,473 | 26,429 | 25,637
Other investments |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Net investment income earned | 251 | 284 | 632 | 515
Investment expenses |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Investment expenses | $ (5,044) | $ (4,447) | $ (15,292) | $ (13,790)

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