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Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Segment Reporting Information [Line Items]        
Total revenues $ 1,584.0 $ 1,657.8 $ 3,450.9 $ 3,648.4
Unites States [Member]        
Segment Reporting Information [Line Items]        
Total revenues 1,086.1 1,176.7 2,486.5 2,754.0
United Kingdom [Member]        
Segment Reporting Information [Line Items]        
Total revenues 269.1 258.7 535.0 472.6
Australia [Member]        
Segment Reporting Information [Line Items]        
Total revenues 81.6 78.8 146.3 147.3
Canada [Member]        
Segment Reporting Information [Line Items]        
Total revenues 59.7 59.5 116.9 117.9
New Zealand [Member]        
Segment Reporting Information [Line Items]        
Total revenues 41.2 45.3 72.0 79.6
Other Foreign [Member]        
Segment Reporting Information [Line Items]        
Total revenues 46.3 38.8 94.2 77.0
Brokerage [Member] | Operating Segments [Member]        
Segment Reporting Information [Line Items]        
Total revenues 1,201.1 1,131.2 2,636.7 2,513.1
Brokerage [Member] | Unites States [Member] | Operating Segments [Member]        
Segment Reporting Information [Line Items]        
Total revenues 740.8 685.3 1,746.7 1,691.8
Brokerage [Member] | United Kingdom [Member] | Operating Segments [Member]        
Segment Reporting Information [Line Items]        
Total revenues 260.9 248.7 515.4 452.4
Brokerage [Member] | Australia [Member] | Operating Segments [Member]        
Segment Reporting Information [Line Items]        
Total revenues 56.9 58.8 100.0 104.8
Brokerage [Member] | Canada [Member] | Operating Segments [Member]        
Segment Reporting Information [Line Items]        
Total revenues 58.2 58.4 114.4 115.6
Brokerage [Member] | New Zealand [Member] | Operating Segments [Member]        
Segment Reporting Information [Line Items]        
Total revenues 38.0 41.2 66.0 71.5
Brokerage [Member] | Other Foreign [Member] | Operating Segments [Member]        
Segment Reporting Information [Line Items]        
Total revenues 46.3 38.8 94.2 77.0
Risk Management [Member] | Operating Segments [Member]        
Segment Reporting Information [Line Items]        
Total revenues 223.2 242.1 472.7 478.5
Risk Management [Member] | Unites States [Member] | Operating Segments [Member]        
Segment Reporting Information [Line Items]        
Total revenues 185.6 206.9 398.3 405.4
Risk Management [Member] | United Kingdom [Member] | Operating Segments [Member]        
Segment Reporting Information [Line Items]        
Total revenues 8.2 10.0 19.6 20.2
Risk Management [Member] | Australia [Member] | Operating Segments [Member]        
Segment Reporting Information [Line Items]        
Total revenues 24.7 20.0 46.3 42.5
Risk Management [Member] | Canada [Member] | Operating Segments [Member]        
Segment Reporting Information [Line Items]        
Total revenues 1.5 1.1 2.5 2.3
Risk Management [Member] | New Zealand [Member] | Operating Segments [Member]        
Segment Reporting Information [Line Items]        
Total revenues 3.2 4.1 6.0 8.1
Corporate [Member] | Operating Segments [Member]        
Segment Reporting Information [Line Items]        
Total revenues 159.7 284.5 341.5 656.8
Corporate [Member] | Unites States [Member] | Operating Segments [Member]        
Segment Reporting Information [Line Items]        
Total revenues $ 159.7 $ 284.5 $ 341.5 $ 656.8