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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.