Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Segment Reporting Information [Line Items]
Total revenues	$ 2,775.4	$ 2,441.9	$ 6,032.1	$ 5,148.0
Earnings before income taxes	365.6	297.4	1,138.3	903.2
Identifiable assets at June 30, 2024 and 2023	63,007.7	54,365.2	63,007.7	54,365.2	$ 51,615.8
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues	2,376.3	2,088.4	5,241.2	4,463.6
Earnings before income taxes	446.3	390.5	1,322.4	1,081.4
Identifiable assets at June 30, 2024 and 2023	58,776.7	50,623.8	58,776.7	50,623.8
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues	398.0	353.6	789.4	684.4
Earnings before income taxes	65.4	49.9	118.6	95.4
Identifiable assets at June 30, 2024 and 2023	1,683.4	1,212.8	1,683.4	1,212.8
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	1.1	(0.1)	1.5
Earnings before income taxes	(146.1)	(143.0)	(302.7)	(273.6)
Identifiable assets at June 30, 2024 and 2023	$ 2,547.6	$ 2,528.6	$ 2,547.6	$ 2,528.6