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Investments in debt and equity securities - Additional Information (Details) $ in Millions | Sep. 30, 2025 USD ($) investment | Dec. 31, 2024 USD ($)
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Investments, Debt and Equity Securities [Abstract] |  |
Net unrealized investment gains on equity securities held | $ | $ 32.4 | $ 23.2
Number of investments in an unrealized loss position | 193 |
Number of investments in an unrealized loss positions for more than 12 months | 150 |

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- Definition Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without an allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (b)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-4 + Details
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