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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Goodwill [Line Items] |  |
Total goodwill - net | $ 4,160.4 | $ 3,767.8
Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill - net | 2,311.3 | 2,138.5
United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill - net | 745.6 | 666.5
Canada [Member] |  |
Goodwill [Line Items] |  |
Total goodwill - net | 345.9 | 292.2
Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill - net | 427.8 | 382.7
New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill - net | 226.7 | 205.3
Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill - net | 103.1 | 82.6
Brokerage [Member] |  |
Goodwill [Line Items] |  |
Total goodwill - net | 4,114.5 | 3,736.9
Brokerage [Member] | Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill - net | 2,285.5 | 2,115.0
Brokerage [Member] | United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill - net | 738.3 | 662.2
Brokerage [Member] | Canada [Member] |  |
Goodwill [Line Items] |  |
Total goodwill - net | 345.9 | 292.2
Brokerage [Member] | Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill - net | 427.8 | 382.7
Brokerage [Member] | New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill - net | 216.8 | 205.0
Brokerage [Member] | Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill - net | 100.2 | 79.8
Risk Management [Member] |  |
Goodwill [Line Items] |  |
Total goodwill - net | 43.0 | 28.1
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 | 7.3 | 4.3
Risk Management [Member] | New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill - net | 9.9 | 0.3
Corporate [Member] |  |
Goodwill [Line Items] |  |
Total goodwill - net | 2.9 | 2.8
Corporate [Member] | Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill - net | $ 2.9 | $ 2.8

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