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