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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Goodwill [Line Items] |  |  |  |
Total goodwill - net | $ 4,625.6 | $ 4,164.8 | [1] | $ 3,753.2
Unites States [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | 2,744.9 | 2,306.7 |  |
United Kingdom [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | 762.9 | 745.7 |  |
Canada [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | 378.6 | 374.0 |  |
Australia [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | 406.6 | 416.6 |  |
New Zealand [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | 219.8 | 218.9 |  |
Other Foreign [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | 112.8 | 102.9 |  |
Brokerage [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | 4,573.6 | 4,119.2 |  | 3,722.3
Brokerage [Member] | Unites States [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | 2,715.3 | 2,280.9 |  |
Brokerage [Member] | United Kingdom [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | 753.7 | 738.5 |  |
Brokerage [Member] | Canada [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | 378.6 | 374.0 |  |
Brokerage [Member] | Australia [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | 406.3 | 416.6 |  |
Brokerage [Member] | New Zealand [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | 209.6 | 209.3 |  |
Brokerage [Member] | Other Foreign [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | 110.1 | 99.9 |  |
Risk Management [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | 49.3 | 42.6 |  | 28.1
Risk Management [Member] | Unites States [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | 29.6 | 25.8 |  |
Risk Management [Member] | United Kingdom [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | 9.2 | 7.2 |  |
Risk Management [Member] | Australia [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | 0.3 |  |  |
Risk Management [Member] | New Zealand [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | 10.2 | 9.6 |  |
Corporate [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | 2.7 | 3.0 |  | $ 2.8
Corporate [Member] | Other Foreign [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Total goodwill - net | $ 2.7 | $ 3.0 |  |
[1] | See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to Topic 606.

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