Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Total revenues	$ 2,044.0	$ 2,138.2	$ 6,522.3	$ 6,236.7
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	1,401.0	1,592.1	4,268.3	4,513.8
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	326.6	288.6	1,216.5	930.0
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	103.0	92.0	304.6	278.7
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	82.3	69.7	266.4	219.9
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	44.6	44.4	133.3	130.0
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues	86.5	51.4	333.2	164.3
Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,736.2	1,499.7	5,599.5	4,500.1
Brokerage [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,140.3	1,000.3	3,492.0	2,919.3
Brokerage [Member] \| United Kingdom [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	315.7	276.5	1,182.1	895.0
Brokerage [Member] \| Australia [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	71.8	62.4	207.4	187.1
Brokerage [Member] \| Canada [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	80.6	68.1	261.9	215.5
Brokerage [Member] \| New Zealand [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	41.3	41.0	122.9	118.9
Brokerage [Member] \| Other Foreign [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	86.5	51.4	333.2	164.3
Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	307.5	280.6	899.4	815.0
Risk Management [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	260.4	233.9	752.9	672.9
Risk Management [Member] \| United Kingdom [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	10.9	12.1	34.4	35.0
Risk Management [Member] \| Australia [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	31.2	29.6	97.2	91.6
Risk Management [Member] \| Canada [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1.7	1.6	4.5	4.4
Risk Management [Member] \| New Zealand [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	3.3	3.4	10.4	11.1
Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	0.3	357.9	23.4	921.6
Corporate [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 0.3	$ 357.9	$ 23.4	$ 921.6