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Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Segment Reporting Information [Line Items]
Total revenues	$ 2,044.0	$ 2,138.2	$ 6,522.3	$ 6,236.7
Earnings before income taxes	315.0	231.4	1,205.0	862.1
Identifiable assets at September 30, 2022 and 2021	38,667.8	26,915.8	38,667.8	26,915.8	$ 33,345.0
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues	1,736.2	1,499.7	5,599.5	4,500.1
Earnings before income taxes	370.7	334.5	1,400.0	1,114.5
Identifiable assets at September 30, 2022 and 2021	34,924.4	1,079.7	34,924.4	1,079.7
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues	307.5	280.6	899.4	815.0
Earnings before income taxes	36.6	29.5	107.7	87.0
Identifiable assets at September 30, 2022 and 2021	1,128.0	21,184.7	1,128.0	21,184.7
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	0.3	357.9	23.4	921.6
Earnings before income taxes	(92.3)	(132.6)	(302.7)	(339.4)
Identifiable assets at September 30, 2022 and 2021	$ 2,615.4	$ 4,651.4	$ 2,615.4	$ 4,651.4

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