Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Total revenues	$ 2,441.9	$ 2,043.0	$ 5,148.0	$ 4,478.3
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	1,498.3	1,265.5	3,253.8	2,867.3
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	539.2	438.7	1,064.0	889.9
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	121.3	110.9	219.5	201.6
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	106.1	95.2	201.3	184.1
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	57.3	50.8	99.2	88.7
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues	119.7	81.9	310.2	246.7
Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	2,088.4	1,740.7	4,463.6	3,863.3
Brokerage [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,199.7	1,015.0	2,675.0	2,351.7
Brokerage [Member] \| United Kingdom [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	527.6	426.2	1,041.1	866.4
Brokerage [Member] \| Australia [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	83.8	76.6	148.0	135.6
Brokerage [Member] \| Canada [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	104.6	93.8	198.3	181.3
Brokerage [Member] \| New Zealand [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	53.0	47.2	91.0	81.6
Brokerage [Member] \| Other Foreign [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	119.7	81.9	310.2	246.7
Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	353.6	302.0	684.4	591.9
Risk Management [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	298.7	250.2	578.8	492.5
Risk Management [Member] \| United Kingdom [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	11.6	12.5	22.9	23.5
Risk Management [Member] \| Australia [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	37.5	34.3	71.5	66.0
Risk Management [Member] \| Canada [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1.5	1.4	3.0	2.8
Risk Management [Member] \| New Zealand [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	4.3	3.6	$ 8.2	7.1
Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	(0.1)	0.3		23.1
Corporate [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	$ (0.1)	$ 0.3		$ 23.1