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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions | Sep. 30, 2025 | Dec. 31, 2024 | Sep. 30, 2024
--- | --- | --- | ---
Goodwill [Line Items] |  |  |
Total goodwill | $ 22,213.3 | $ 12,270.2 | $ 12,193.4
United States |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 16,306.1 | 7,040.4 |
United Kingdom |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 2,921.0 | 2,617.1 |
Canada |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 619.5 | 586.9 |
Australia |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 809.2 | 728.4 |
New Zealand |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 230.3 | 191.7 |
Other foreign |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 1,327.2 | 1,105.7 |
Brokerage |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 21,738.9 | 11,923.4 |
Risk Management |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 456.6 | 328.3 |
Corporate |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 17.8 | 18.5 |
Operating Segments | Brokerage |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 21,738.9 | 11,923.4 | 11,829.3
Operating Segments | Brokerage | United States |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 16,231.3 | 6,965.6 |
Operating Segments | Brokerage | United Kingdom |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 2,780.0 | 2,591.4 |
Operating Segments | Brokerage | Canada |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 619.5 | 586.9 |
Operating Segments | Brokerage | Australia |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 577.2 | 509.1 |
Operating Segments | Brokerage | New Zealand |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 221.5 | 183.2 |
Operating Segments | Brokerage | Other foreign |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 1,309.4 | 1,087.2 |
Operating Segments | Risk Management |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 456.6 | 328.3 | 345.2
Operating Segments | Risk Management | United States |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 74.8 | 74.8 |
Operating Segments | Risk Management | United Kingdom |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 141.0 | 25.7 |
Operating Segments | Risk Management | Canada |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 0.0 | 0.0 |
Operating Segments | Risk Management | Australia |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 232.0 | 219.3 |
Operating Segments | Risk Management | New Zealand |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 8.8 | 8.5 |
Operating Segments | Risk Management | Other foreign |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 0.0 | 0.0 |
Operating Segments | Corporate |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 17.8 | 18.5 | $ 18.9
Operating Segments | Corporate | United States |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 0.0 | 0.0 |
Operating Segments | Corporate | United Kingdom |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 0.0 | 0.0 |
Operating Segments | Corporate | Canada |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 0.0 | 0.0 |
Operating Segments | Corporate | Australia |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 0.0 | 0.0 |
Operating Segments | Corporate | New Zealand |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 0.0 | 0.0 |
Operating Segments | Corporate | Other foreign |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | $ 17.8 | $ 18.5 |

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