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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions | Mar. 31, 2022 | Dec. 31, 2021
--- | --- | ---
Goodwill [Line Items] |  |
Total goodwill | $ 8,727.7 | $ 8,666.2
Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 4,422.0 | 4,473.9
United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 2,239.8 | 2,213.7
Canada [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 593.4 | 572.9
Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 518.5 | 498.1
New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 234.4 | 227.8
Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 719.6 | 679.8
Brokerage [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 8,605.7 | 8,544.6
Brokerage [Member] | Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 4,357.2 | 4,409.1
Brokerage [Member] | United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 2,225.0 | 2,198.7
Brokerage [Member] | Canada [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 593.4 | 572.9
Brokerage [Member] | Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 507.2 | 487.2
Brokerage [Member] | New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 223.9 | 217.6
Brokerage [Member] | Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 699.0 | 659.1
Risk Management [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 101.4 | 100.9
Risk Management [Member] | Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 64.8 | 64.8
Risk Management [Member] | United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 14.8 | 15.0
Risk Management [Member] | Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 11.3 | 10.9
Risk Management [Member] | New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 10.5 | 10.2
Corporate [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 20.6 | 20.7
Corporate [Member] | Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | $ 20.6 | $ 20.7

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