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Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | $ 1,778.5 | $ 1,593.7 | [1] | $ 5,276.6 | $ 4,729.6 | [1]
Operating Segments [Member] | Brokerage [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 1,049.4 | 938.7 |  | 3,245.1 | 2,919.7 |
Operating Segments [Member] | Risk Management [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 236.4 | 224.4 |  | 703.6 | 651.1 |
Operating Segments [Member] | Corporate [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 492.7 | 430.6 |  | 1,327.9 | 1,158.8 |
Unites States [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 1,418.4 | 1,241.5 |  | 4,112.0 | 3,682.5 |
Unites States [Member] | Operating Segments [Member] | Brokerage [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 724.2 | 620.4 |  | 2,192.5 | 1,965.5 |
Unites States [Member] | Operating Segments [Member] | Risk Management [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 201.5 | 190.5 |  | 591.6 | 558.2 |
Unites States [Member] | Operating Segments [Member] | Corporate [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 492.7 | 430.6 |  | 1,327.9 | 1,158.8 |
United Kingdom [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 178.0 | 177.2 |  | 591.1 | 536.8 |
United Kingdom [Member] | Operating Segments [Member] | Brokerage [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 169.3 | 168.8 |  | 564.6 | 514.8 |
United Kingdom [Member] | Operating Segments [Member] | Risk Management [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 8.7 | 8.4 |  | 26.5 | 22.0 |
Australia [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 70.2 | 69.6 |  | 217.1 | 199.7 |
Australia [Member] | Operating Segments [Member] | Brokerage [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 49.1 | 50.5 |  | 146.6 | 142.8 |
Australia [Member] | Operating Segments [Member] | Risk Management [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 21.1 | 19.1 |  | 70.5 | 56.9 |
Canada [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 41.2 | 36.8 |  | 133.9 | 107.2 |
Canada [Member] | Operating Segments [Member] | Brokerage [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 40.3 | 35.4 |  | 130.9 | 104.1 |
Canada [Member] | Operating Segments [Member] | Risk Management [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 0.9 | 1.4 |  | 3.0 | 3.1 |
New Zealand [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 39.2 | 41.0 |  | 116.5 | 110.8 |
New Zealand [Member] | Operating Segments [Member] | Brokerage [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 35.0 | 36.0 |  | 104.5 | 99.9 |
New Zealand [Member] | Operating Segments [Member] | Risk Management [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 4.2 | 5.0 |  | 12.0 | 10.9 |
Other Foreign [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 31.5 | 27.6 |  | 106.0 | 92.6 |
Other Foreign [Member] | Operating Segments [Member] | Brokerage [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | $ 31.5 | $ 27.6 |  | $ 106.0 | $ 92.6 |
[1] | See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to Topic 606.

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