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Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2024 | Mar. 31, 2023
Segment Reporting Information [Line Items] |  |
Total revenues | $ 3,256.7 | $ 2,706.1
Unites States [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 2,134.9 | 1,755.5
United Kingdom [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 598.8 | 524.8
Australia [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 121.0 | 98.2
Canada [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 104.8 | 95.2
New Zealand [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 43.9 | 41.9
Other Foreign [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 253.3 | 190.5
Brokerage [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 2,864.9 | 2,375.2
Brokerage [Member] | Unites States [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 1,814.0 | 1,475.3
Brokerage [Member] | United Kingdom [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 584.8 | 513.5
Brokerage [Member] | Australia [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 68.0 | 64.2
Brokerage [Member] | Canada [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 103.1 | 93.7
Brokerage [Member] | New Zealand [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 41.7 | 38.0
Brokerage [Member] | Other Foreign [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 253.3 | 190.5
Risk Management [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 391.4 | 330.8
Risk Management [Member] | Unites States [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 320.5 | 280.1
Risk Management [Member] | United Kingdom [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 14.0 | 11.3
Risk Management [Member] | Australia [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 53.0 | 34.0
Risk Management [Member] | Canada [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 1.7 | 1.5
Risk Management [Member] | New Zealand [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 2.2 | 3.9
Corporate [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 0.4 | 0.1
Corporate [Member] | Unites States [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | $ 0.4 | $ 0.1

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