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Supplemental Disclosures of Cash Flow Information - Additional Information (Detail) - USD ($) $ in Millions | 1 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jan. 31, 2025 | Sep. 30, 2025 | Sep. 30, 2024 | Dec. 31, 2024 | Dec. 31, 2023
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Proceeds from issuance of common stock |  | $ 1,481.3 | $ 138.0 |  |
Money Market Accounts |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Total cash and cash equivalents, restricted cash and fiduciary cash |  | 2,256.2 | 2,687.6 |  |
Proceeds from AssuredPartners financing |  | 13,500.0 |  |  |
Money Market Accounts | Interest income, premium finance revenues and other income |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Increases dividend income |  | 319.9 |  |  |
Proceeds from AssuredPartners financing |  | 363.3 |  |  |
Dividend income |  | $ 647.2 | 327.3 |  |
Money Market Accounts | Over-Allotment Option |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Proceeds from issuance of common stock | $ 1,300.0 |  |  |  |
Qualified Contributory Savings and Thrift 401(k) Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Matching contributions by employer, percentage |  | 100.00% |  |  |
Percentage of eligible compensation for matching contributions by employer |  | 5.00% |  |  |
Matching contributions vesting schedule |  | 5 years |  |  |
Contribution expense to plan |  | $ 87.3 | $ 81.7 |  |
Employer match on eligible compensation |  |  |  | 5.00% | 5.00%

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