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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | --- | ---
Goodwill [Line Items] |  |  |
Total goodwill - net | $ 6,127.0 | $ 5,618.5 | $ 4,625.6
Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 3,433.0 | 3,196.9 |
United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 1,343.4 | 1,190.7 |
Canada [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 492.9 | 454.4 |
Australia [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 473.6 | 427.0 |
New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 232.6 | 218.1 |
Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 151.5 | 131.4 |
Brokerage [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 6,053.6 | 5,548.9 | 4,573.6
Brokerage [Member] | Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 3,399.8 | 3,163.8 |
Brokerage [Member] | United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 1,328.3 | 1,177.8 |
Brokerage [Member] | Canada [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 492.9 | 454.4 |
Brokerage [Member] | Australia [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 462.1 | 416.5 |
Brokerage [Member] | New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 221.9 | 208.0 |
Brokerage [Member] | Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 148.6 | 128.4 |
Risk Management [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 70.5 | 66.6 | 49.3
Risk Management [Member] | Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 33.2 | 33.1 |
Risk Management [Member] | United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 15.1 | 12.9 |
Risk Management [Member] | Australia [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 11.5 | 10.5 |
Risk Management [Member] | New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 10.7 | 10.1 |
Corporate [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 2.9 | 3.0 | $ 2.7
Corporate [Member] | Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | $ 2.9 | $ 3.0 |

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