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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions | Sep. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Goodwill [Line Items] |  |
Total goodwill | $ 10,469.4 | $ 9,489.4
Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 5,823.1 | 5,140.5
United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 2,340.7 | 2,197.9
Canada [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 591.1 | 569.7
Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 489.3 | 477.8
New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 200.5 | 213.3
Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 1,024.7 | 890.2
Brokerage [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 10,338.9 | 9,358.1
Risk Management [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 111.5 | 112.2
Corporate [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 19.0 | 19.1
Operating Segments [Member] | Brokerage [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 10,338.9 | 9,358.1
Operating Segments [Member] | Brokerage [Member] | Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 5,748.3 | 5,065.7
Operating Segments [Member] | Brokerage [Member] | United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 2,322.8 | 2,180.2
Operating Segments [Member] | Brokerage [Member] | Canada [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 591.1 | 569.7
Operating Segments [Member] | Brokerage [Member] | Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 479.5 | 467.6
Operating Segments [Member] | Brokerage [Member] | New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 191.5 | 203.8
Operating Segments [Member] | Brokerage [Member] | Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 1,005.7 | 871.1
Operating Segments [Member] | Risk Management [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 111.5 | 112.2
Operating Segments [Member] | Risk Management [Member] | Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 74.8 | 74.8
Operating Segments [Member] | Risk Management [Member] | United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 17.9 | 17.7
Operating Segments [Member] | Risk Management [Member] | Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 9.8 | 10.2
Operating Segments [Member] | Risk Management [Member] | New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 9.0 | 9.5
Operating Segments [Member] | Corporate [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 19.0 | 19.1
Operating Segments [Member] | Corporate [Member] | Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | $ 19.0 | $ 19.1

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