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Schedule of Total Investment Return (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022
Investments, Debt and Equity Securities [Abstract] |  |  |  |  |
Net investment income |  | $ 14,200 | $ 8,389 | $ 39,424 | $ 16,911
Net realized investment losses |  | (133) | 2,234 | (2,414) | (33,067)
Change in unrealized holding gains (losses) |  | (1,254) | (23,037) | 6,108 | (64,369)
Net realized and unrealized investment returns |  | (1,387) | (20,803) | 3,694 | (97,436)
Total investment return |  | $ 12,813 | $ (12,414) | $ 43,118 | $ (80,525)
Total investment return % | [1] | 0.90% | (0.90%) | 3.20% | (5.60%)
Average investment portfolio | [2] | $ 1,355,077 | $ 1,341,285 | $ 1,354,727 | $ 1,444,037
[1] | Not annualized.
[2] | Average of total cash and invested assets, net of receivable/payable for securities purchased and sold, as of the beginning and end of the period.

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