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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Goodwill [Line Items] |  |  |
Total goodwill - net | $ 4,501.3 | $ 4,164.8 | [1]
Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 2,656.3 | 2,306.7 |
United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 733.7 | 745.7 |
Canada [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 373.2 | 374.0 |
Australia [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 411.8 | 416.6 |
New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 207.5 | 218.9 |
Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 118.8 | 102.9 |
Brokerage [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 4,452.0 | 4,119.2 |
Brokerage [Member] | Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 2,626.7 | 2,280.9 |
Brokerage [Member] | United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 726.7 | 738.5 |
Brokerage [Member] | Canada [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 373.2 | 374.0 |
Brokerage [Member] | Australia [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 411.8 | 416.6 |
Brokerage [Member] | New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 197.4 | 209.3 |
Brokerage [Member] | Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 116.2 | 99.9 |
Risk Management [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 46.7 | 42.6 |
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 | 7.0 | 7.2 |
Risk Management [Member] | New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 10.1 | 9.6 |
Corporate [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 2.6 | 3.0 |
Corporate [Member] | Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | $ 2.6 | $ 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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