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Schedule of Total Investment Return (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Investments, Debt and Equity Securities [Abstract] |  |  |  |  |  |  |  |
Net investment income |  | $ 17,911 | $ 16,488 |  | $ 47,400 |  | $ 46,319 |
Net realized investment gains (losses) |  | (3,994) | (512) |  | (3,731) |  | 540 |
Change in unrealized holding gains (losses) |  | 571 | 12,787 |  | 7,787 |  | 18,721 |
Net realized and unrealized investment returns |  | (3,423) | 12,275 |  | 4,056 |  | 19,261 |
Total investment return |  | $ 14,488 | $ 28,763 |  | $ 51,456 |  | $ 65,580 |
Total investment return % | [1] | 1.00% | 2.00% |  | 3.60% |  | 4.60% |
Average investment portfolio |  | $ 1,434,105 | $ 1,451,641 | [2] | $ 1,437,944 | [2] | $ 1,429,253 | [2]
[1] | Not annualized.
[2] | Average of total cash and invested assets, net of receivable/payable for securities, as of the beginning and end of the period.

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