Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Total revenues	$ 2,492.0	$ 2,044.0	$ 7,640.0	$ 6,522.3
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	1,662.7	1,401.0	4,916.5	4,268.3
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	439.2	326.6	1,503.2	1,216.5
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	118.3	103.0	337.8	304.6
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	91.6	82.3	292.9	266.4
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	53.6	44.6	152.8	133.3
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues	126.6	86.5	436.8	333.2
Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	2,122.1	1,736.2	6,585.7	5,599.5
Brokerage [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,349.0	1,140.3	4,024.0	3,492.0
Brokerage [Member] \| United Kingdom [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	427.3	315.7	1,468.4	1,182.1
Brokerage [Member] \| Australia [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	79.8	71.8	227.8	207.4
Brokerage [Member] \| Canada [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	89.8	80.6	288.1	261.9
Brokerage [Member] \| New Zealand [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	49.6	41.3	140.6	122.9
Brokerage [Member] \| Other Foreign [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	126.6	86.5	436.8	333.2
Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	369.4	307.5	1,053.8	899.4
Risk Management [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	313.2	260.4	892.0	752.9
Risk Management [Member] \| United Kingdom [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	11.9	10.9	34.8	34.4
Risk Management [Member] \| Australia [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	38.5	31.2	110.0	97.2
Risk Management [Member] \| Canada [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1.8	1.7	4.8	4.5
Risk Management [Member] \| New Zealand [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	4.0	3.3	12.2	10.4
Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	0.5	0.3	0.5	23.4
Corporate [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 0.5	$ 0.3	$ 0.5	$ 23.4