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Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
Segment Reporting Information [Line Items]
Total revenues	$ 2,441.9	$ 2,043.0	$ 5,148.0	$ 4,478.3
Earnings before income taxes	297.4	352.3	903.2	890.0
Identifiable assets at June 30, 2023 and 2022	54,365.2	39,850.8	54,365.2	39,850.8	$ 38,358.4
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues	2,088.4	1,740.7	4,463.6	3,863.3
Earnings before income taxes	390.5	410.9	1,081.4	1,029.3
Identifiable assets at June 30, 2023 and 2022	50,623.8	36,574.3	50,623.8	36,574.3
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues	353.6	302.0	684.4	591.9
Earnings before income taxes	49.9	38.8	95.4	71.1
Identifiable assets at June 30, 2023 and 2022	1,212.8	1,120.3	1,212.8	1,120.3
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	(0.1)	0.3		23.1
Earnings before income taxes	(143.0)	(97.4)	(273.6)	(210.4)
Identifiable assets at June 30, 2023 and 2022	$ 2,528.6	$ 2,156.2	$ 2,528.6	$ 2,156.2

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