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Schedule of Total Investment Return (Detail) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2025 | Jun. 30, 2024 | Jun. 30, 2025 | Jun. 30, 2024
Investments, Debt and Equity Securities [Abstract] |  |  |  |  |  |  |  |
Net investment income |  | $ 14,707 | $ 15,311 |  | $ 29,489 |  | $ 29,831 |
Net realized investment gains |  | 127 | 205 |  | 263 |  | 1,052 |
Change in unrealized holding gains (losses) |  | 2,833 | 2,387 |  | 7,216 |  | 5,934 |
Net realized and unrealized investment returns |  | 2,960 | 2,592 |  | 7,479 |  | 6,986 |
Total investment return |  | $ 17,667 | $ 17,903 |  | $ 36,968 |  | $ 36,817 |
Total investment return % | [1] | 1.20% | 1.30% |  | 2.60% |  | 2.60% |
Average investment portfolio |  | $ 1,432,379 | $ 1,426,266 | [2] | $ 1,436,827 | [2] | $ 1,412,821 | [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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