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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Goodwill [Line Items] |  |  |
Total goodwill - net | $ 5,618.5 | $ 4,625.6 | $ 4,164.8
Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 3,196.9 | 2,744.9 |
United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 1,190.7 | 762.9 |
Canada [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 454.4 | 378.6 |
Australia [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 427.0 | 406.6 |
New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 218.1 | 219.8 |
Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 131.4 | 112.8 |
Brokerage [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 5,548.9 | 4,573.6 | 4,119.2
Brokerage [Member] | Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 3,163.8 | 2,715.3 |
Brokerage [Member] | United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 1,177.8 | 753.7 |
Brokerage [Member] | Canada [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 454.4 | 378.6 |
Brokerage [Member] | Australia [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 416.5 | 406.3 |
Brokerage [Member] | New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 208.0 | 209.6 |
Brokerage [Member] | Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 128.4 | 110.1 |
Risk Management [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 66.6 | 49.3 | 42.6
Risk Management [Member] | Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 33.1 | 29.6 |
Risk Management [Member] | United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 12.9 | 9.2 |
Risk Management [Member] | Australia [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 10.5 | 0.3 |
Risk Management [Member] | New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 10.1 | 10.2 |
Corporate [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 3.0 | 2.7 | $ 3.0
Corporate [Member] | Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | $ 3.0 | $ 2.7 |

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