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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Goodwill [Line Items] |  |  |
Total goodwill - net | $ 4,197.9 | $ 3,767.8 | $ 3,662.9
Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 2,340.0 | 2,138.5 |
United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 745.7 | 666.5 |
Canada [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 375.3 | 292.2 |
Australia [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 416.6 | 382.7 |
New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 218.9 | 205.3 |
Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 101.4 | 82.6 |
Brokerage [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 4,152.3 | 3,736.9 | 3,635.6
Brokerage [Member] | Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 2,314.2 | 2,115.0 |
Brokerage [Member] | United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 738.5 | 662.2 |
Brokerage [Member] | Canada [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 375.3 | 292.2 |
Brokerage [Member] | Australia [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 416.6 | 382.7 |
Brokerage [Member] | New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 209.3 | 205.0 |
Brokerage [Member] | Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 98.4 | 79.8 |
Risk Management [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 42.6 | 28.1 | $ 27.3
Risk Management [Member] | Unites States [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 25.8 | 23.5 |
Risk Management [Member] | United Kingdom [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 7.2 | 4.3 |
Risk Management [Member] | New Zealand [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 9.6 | 0.3 |
Corporate [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | 3.0 | 2.8 |
Corporate [Member] | Other Foreign [Member] |  |  |
Goodwill [Line Items] |  |  |
Total goodwill - net | $ 3.0 | $ 2.8 |

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