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Investments - Net Investment Income (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2024 | Mar. 31, 2023
Net Investment Income [Line Items] |  |
Net investment income earned | $ 107,849 | $ 91,506
Fixed income securities |  |
Net Investment Income [Line Items] |  |
Net investment income earned | 94,102 | 80,087
Commercial mortgage loans ("CMLs") |  |
Net Investment Income [Line Items] |  |
Net investment income earned | 2,794 | 1,965
Equity securities |  |
Net Investment Income [Line Items] |  |
Net investment income earned | 4,908 | 1,205
Short-term investments |  |
Net Investment Income [Line Items] |  |
Net investment income earned | 3,519 | 4,650
Alternative investments |  |
Net Investment Income [Line Items] |  |
Net investment income earned | 6,881 | 7,768
Other investments |  |
Net Investment Income [Line Items] |  |
Net investment income earned | 263 | 43
Investment expenses |  |
Net Investment Income [Line Items] |  |
Investment expenses | $ (4,618) | $ (4,212)

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