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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Goodwill [Line Items] |  |
Total goodwill | $ 5,832.2 | $ 5,618.5
Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 3,300.2 | 3,196.9
United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 1,274.3 | 1,190.7
Canada [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 450.3 | 454.4
Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 444.6 | 427.0
New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 216.7 | 218.1
Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 146.1 | 131.4
Brokerage [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 5,761.4 | 5,548.9
Brokerage [Member] | Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 3,267.0 | 3,163.8
Brokerage [Member] | United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 1,260.4 | 1,177.8
Brokerage [Member] | Canada [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 450.3 | 454.4
Brokerage [Member] | Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 433.8 | 416.5
Brokerage [Member] | New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 206.7 | 208.0
Brokerage [Member] | Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 143.2 | 128.4
Risk Management [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 67.9 | 66.6
Risk Management [Member] | Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 33.2 | 33.1
Risk Management [Member] | United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 13.9 | 12.9
Risk Management [Member] | Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 10.8 | 10.5
Risk Management [Member] | New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 10.0 | 10.1
Corporate [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 2.9 | 3.0
Corporate [Member] | Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | $ 2.9 | $ 3.0

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