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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Goodwill [Line Items] |  |
Total goodwill | $ 6,627.5 | $ 6,127.0
Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 3,633.7 | 3,433.0
United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 1,621.8 | 1,343.4
Canada [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 523.0 | 492.9
Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 461.0 | 473.6
New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 231.1 | 232.6
Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 156.9 | 151.5
Brokerage [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 6,522.8 | 6,053.6
Brokerage [Member] | Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 3,568.9 | 3,399.8
Brokerage [Member] | United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 1,606.4 | 1,328.3
Brokerage [Member] | Canada [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 523.0 | 492.9
Brokerage [Member] | Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 450.0 | 462.1
Brokerage [Member] | New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 220.5 | 221.9
Brokerage [Member] | Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 154.0 | 148.6
Risk Management [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 101.8 | 70.5
Risk Management [Member] | Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 64.8 | 33.2
Risk Management [Member] | United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 15.4 | 15.1
Risk Management [Member] | Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 11.0 | 11.5
Risk Management [Member] | New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 10.6 | 10.7
Corporate [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 2.9 | 2.9
Corporate [Member] | Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | $ 2.9 | $ 2.9

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