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Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2022 | Mar. 31, 2021
Segment Reporting Information [Line Items] |  |
Total revenues | $ 2,435.3 | $ 2,165.0
Unites States [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 1,601.8 | 1,616.2
United Kingdom [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 451.2 | 303.5
Australia [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 90.7 | 83.0
Canada [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 88.9 | 68.9
New Zealand [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 37.9 | 36.2
Other Foreign [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 164.8 | 57.2
Brokerage [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 2,122.6 | 1,610.2
Brokerage [Member] | Unites States [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 1,336.7 | 1,106.6
Brokerage [Member] | United Kingdom [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 440.2 | 292.4
Brokerage [Member] | Australia [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 59.0 | 54.0
Brokerage [Member] | Canada [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 87.5 | 67.5
Brokerage [Member] | New Zealand [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 34.4 | 32.5
Brokerage [Member] | Other Foreign [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 164.8 | 57.2
Risk Management [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 289.9 | 252.7
Risk Management [Member] | Unites States [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 242.3 | 207.5
Risk Management [Member] | United Kingdom [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 11.0 | 11.1
Risk Management [Member] | Australia [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 31.7 | 29.0
Risk Management [Member] | Canada [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 1.4 | 1.4
Risk Management [Member] | New Zealand [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 3.5 | 3.7
Corporate [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 22.8 | 302.1
Corporate [Member] | Unites States [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Total revenues | $ 22.8 | $ 302.1

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