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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
--- | --- | --- | ---
Goodwill [Line Items] |  |  |
Total goodwill - net | $ 11,475.6 | $ 9,489.4 | $ 8,666.2
Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 6,379.3 | 5,140.5 |
United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 2,511.9 | 2,197.9 |
Canada [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 623.7 | 569.7 |
Australia [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 650.5 | 477.8 |
New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 213.8 | 213.3 |
Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 1,096.4 | 890.2 |
Brokerage [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 11,217.8 | 9,358.1 | 8,544.6
Risk Management [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 238.8 | 112.2 | 100.9
Corporate [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 19.0 | 19.1 | 20.7
Operating Segments [Member] | Brokerage [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 11,217.8 | 9,358.1 | 8,544.6
Operating Segments [Member] | Brokerage [Member] | Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 6,304.5 | 5,065.7 |
Operating Segments [Member] | Brokerage [Member] | United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 2,493.4 | 2,180.2 |
Operating Segments [Member] | Brokerage [Member] | Canada [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 623.7 | 569.7 |
Operating Segments [Member] | Brokerage [Member] | Australia [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 514.6 | 467.6 |
Operating Segments [Member] | Brokerage [Member] | New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 204.2 | 203.8 |
Operating Segments [Member] | Brokerage [Member] | Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 1,077.4 | 871.1 |
Operating Segments [Member] | Risk Management [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 238.8 | 112.2 | 100.9
Operating Segments [Member] | Risk Management [Member] | Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 74.8 | 74.8 |
Operating Segments [Member] | Risk Management [Member] | United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 18.5 | 17.7 |
Operating Segments [Member] | Risk Management [Member] | Australia [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 135.9 | 10.2 |
Operating Segments [Member] | Risk Management [Member] | New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 9.6 | 9.5 |
Operating Segments [Member] | Corporate [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 19.0 | 19.1 | $ 20.7
Operating Segments [Member] | Corporate [Member] | Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | $ 19.0 | $ 19.1 |

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