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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions | Sep. 30, 2022 | Dec. 31, 2021
--- | --- | ---
Goodwill [Line Items] |  |
Total goodwill | $ 8,837.2 | $ 8,666.2
Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 4,712.8 | 4,473.9
United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 2,056.6 | 2,213.7
Canada [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 581.0 | 572.9
Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 459.5 | 498.1
New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 198.5 | 227.8
Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 828.8 | 679.8
Brokerage [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 8,707.1 | 8,544.6
Brokerage [Member] | Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 4,638.0 | 4,409.1
Brokerage [Member] | United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 2,040.1 | 2,198.7
Brokerage [Member] | Canada [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 581.0 | 572.9
Brokerage [Member] | Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 449.4 | 487.2
Brokerage [Member] | New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 189.6 | 217.6
Brokerage [Member] | Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 809.0 | 659.1
Risk Management [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 110.3 | 100.9
Risk Management [Member] | Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 74.8 | 64.8
Risk Management [Member] | United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 16.5 | 15.0
Risk Management [Member] | Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 10.1 | 10.9
Risk Management [Member] | New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 8.9 | 10.2
Corporate [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 19.8 | 20.7
Corporate [Member] | Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | $ 19.8 | $ 20.7

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