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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | --- | ---
Goodwill [Line Items] |  |  |
Total goodwill - net | $ 8,666.2 | $ 6,127.0 | $ 5,618.5
Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 4,473.9 | 3,433.0 |
United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 2,213.7 | 1,343.4 |
Canada [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 572.9 | 492.9 |
Australia [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 498.1 | 473.6 |
New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 227.8 | 232.6 |
Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 679.8 | 151.5 |
Brokerage [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 8,544.6 | 6,053.6 | 5,548.9
Brokerage [Member] | Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 4,409.1 | 3,399.8 |
Brokerage [Member] | United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 2,198.7 | 1,328.3 |
Brokerage [Member] | Canada [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 572.9 | 492.9 |
Brokerage [Member] | Australia [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 487.2 | 462.1 |
Brokerage [Member] | New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 217.6 | 221.9 |
Brokerage [Member] | Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 659.1 | 148.6 |
Risk Management [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 100.9 | 70.5 | 66.6
Risk Management [Member] | Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 64.8 | 33.2 |
Risk Management [Member] | United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 15.0 | 15.1 |
Risk Management [Member] | Australia [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 10.9 | 11.5 |
Risk Management [Member] | New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 10.2 | 10.7 |
Corporate [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 20.7 | 2.9 | $ 3.0
Corporate [Member] | Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | $ 20.7 | $ 2.9 |

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