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Supplemental Disclosures of Cash Flow Information - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Net operating losses in U.K. additional cash tax payments |  |  | $ 49.0 | $ 28.4
Cash and cash equivalents - non-restricted cash | $ 14,758.7 | $ 780.9 | $ 539.4 |
Stock issuance | 8,344.4 |  |  |
Money Market Accounts [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total cash and cash equivalents, restricted cash and fiduciary cash | 15,371.6 | 1,744.9 |  |
Stock issuance | 8,500.0 |  |  |
Senior notes | 5,000.0 |  |  |
Money Market Accounts [Member] | Investment Income [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Increases dividend income | 105.9 |  |  |
Proceeds from AssuredPartners financing | 29.0 |  |  |
Dividend income | $ 473.2 | $ 367.3 |  |
Qualified Contributory Savings and Thrift 401(k) Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Matching contributions by employer, percentage | 100.00% |  |  |
Percentage of eligible compensation for matching contributions by employer | 5.00% | 5.00% | 5.00% |
Matching contributions vesting schedule | 5 years |  |  |
Contribution expense to plan | $ 105.4 | $ 86.0 | $ 73.8 |
Employer match on eligible compensation | 5.00% | 5.00% |  |

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