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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
--- | --- | --- | ---
Goodwill [Line Items] |  |  |
Total goodwill - net | $ 9,489.4 | $ 8,666.2 | $ 6,127.0
Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 5,140.5 | 4,473.9 |
United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 2,197.9 | 2,213.7 |
Canada [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 569.7 | 572.9 |
Australia [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 477.8 | 498.1 |
New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 213.3 | 227.8 |
Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 890.2 | 679.8 |
Brokerage [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 9,358.1 | 8,544.6 | 6,053.6
Brokerage [Member] | Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 5,065.7 | 4,409.1 |
Brokerage [Member] | United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 2,180.2 | 2,198.7 |
Brokerage [Member] | Canada [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 569.7 | 572.9 |
Brokerage [Member] | Australia [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 467.6 | 487.2 |
Brokerage [Member] | New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 203.8 | 217.6 |
Brokerage [Member] | Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 871.1 | 659.1 |
Risk Management [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 112.2 | 100.9 | 70.5
Risk Management [Member] | Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 74.8 | 64.8 |
Risk Management [Member] | United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 17.7 | 15.0 |
Risk Management [Member] | Australia [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 10.2 | 10.9 |
Risk Management [Member] | New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 9.5 | 10.2 |
Corporate [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 19.1 | 20.7 | $ 2.9
Corporate [Member] | Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | $ 19.1 | $ 20.7 |

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