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v3.23.1

Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2023 | Mar. 31, 2022
Segment Reporting Information [Line Items] |  |
Total revenues | $ 2,706.1 | $ 2,435.3
Unites States [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 1,755.5 | 1,601.8
United Kingdom [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 524.8 | 451.2
Australia [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 98.2 | 90.7
Canada [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 95.2 | 88.9
New Zealand [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 41.9 | 37.9
Other Foreign [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 190.5 | 164.8
Brokerage [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 2,375.2 | 2,122.6
Brokerage [Member] | Unites States [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 1,475.3 | 1,336.7
Brokerage [Member] | United Kingdom [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 513.5 | 440.2
Brokerage [Member] | Australia [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 64.2 | 59.0
Brokerage [Member] | Canada [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 93.7 | 87.5
Brokerage [Member] | New Zealand [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 38.0 | 34.4
Brokerage [Member] | Other Foreign [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 190.5 | 164.8
Risk Management [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 330.8 | 289.9
Risk Management [Member] | Unites States [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 280.1 | 242.3
Risk Management [Member] | United Kingdom [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 11.3 | 11.0
Risk Management [Member] | Australia [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 34.0 | 31.7
Risk Management [Member] | Canada [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 1.5 | 1.4
Risk Management [Member] | New Zealand [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 3.9 | 3.5
Corporate [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 0.1 | 22.8
Corporate [Member] | Unites States [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | $ 0.1 | $ 22.8

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