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