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Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | $ 1,660.4 | $ 1,489.5 | [1] | $ 3,498.1 | $ 3,135.9 | [1]
Operating Segments [Member] | Brokerage [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 1,000.1 | 896.1 |  | 2,195.7 | 1,981.0 |
Operating Segments [Member] | Risk Management [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 237.3 | 216.8 |  | 467.2 | 426.7 |
Operating Segments [Member] | Corporate [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 423.0 | 376.6 |  | 835.2 | 728.2 |
Unites States [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 1,235.8 | 1,114.0 |  | 2,693.6 | 2,441.0 |
Unites States [Member] | Operating Segments [Member] | Brokerage [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 615.5 | 552.1 |  | 1,468.3 | 1,345.1 |
Unites States [Member] | Operating Segments [Member] | Risk Management [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 197.3 | 185.3 |  | 390.1 | 367.7 |
Unites States [Member] | Operating Segments [Member] | Corporate [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 423.0 | 376.6 |  | 835.2 | 728.2 |
United Kingdom [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 213.2 | 191.9 |  | 413.1 | 359.6 |
United Kingdom [Member] | Operating Segments [Member] | Brokerage [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 204.4 | 184.7 |  | 395.4 | 346.0 |
United Kingdom [Member] | Operating Segments [Member] | Risk Management [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 8.8 | 7.2 |  | 17.7 | 13.6 |
Australia [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 79.6 | 71.1 |  | 146.9 | 130.1 |
Australia [Member] | Operating Segments [Member] | Brokerage [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 53.9 | 52.0 |  | 97.5 | 92.3 |
Australia [Member] | Operating Segments [Member] | Risk Management [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 25.7 | 19.1 |  | 49.4 | 37.8 |
Canada [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 49.0 | 37.5 |  | 92.8 | 70.4 |
Canada [Member] | Operating Segments [Member] | Brokerage [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 47.8 | 36.6 |  | 90.6 | 68.7 |
Canada [Member] | Operating Segments [Member] | Risk Management [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 1.2 | 0.9 |  | 2.2 | 1.7 |
New Zealand [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 43.7 | 41.1 |  | 77.3 | 69.8 |
New Zealand [Member] | Operating Segments [Member] | Brokerage [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 39.4 | 36.8 |  | 69.5 | 63.9 |
New Zealand [Member] | Operating Segments [Member] | Risk Management [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 4.3 | 4.3 |  | 7.8 | 5.9 |
Other Foreign [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | 39.1 | 33.9 |  | 74.4 | 65.0 |
Other Foreign [Member] | Operating Segments [Member] | Brokerage [Member] |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |
Total revenues | $ 39.1 | $ 33.9 |  | $ 74.4 | $ 65.0 |
[1] | See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to ASC 606.

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