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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, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021
Segment Reporting Information [Line Items]
Total revenues	$ 2,043.0	$ 1,933.5	$ 4,478.3	$ 4,098.5
Earnings before income taxes	352.3	222.3	890.0	630.7
Identifiable assets at June 30, 2022 and 2021	39,990.4	27,817.2	39,990.4	27,817.2	$ 33,345.0
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues	1,740.7	1,390.2	3,863.3	3,000.4
Earnings before income taxes	410.9	299.7	1,029.3	780.0
Identifiable assets at June 30, 2022 and 2021	36,374.1	21,858.6	36,374.1	21,858.6
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues	302.0	281.7	591.9	534.4
Earnings before income taxes	38.8	33.4	71.1	57.5
Identifiable assets at June 30, 2022 and 2021	1,101.8	1,058.5	1,101.8	1,058.5
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	0.3	261.6	23.1	563.7
Earnings before income taxes	(97.4)	(110.8)	(210.4)	(206.8)
Identifiable assets at June 30, 2022 and 2021	$ 2,514.5	$ 4,900.1	$ 2,514.5	$ 4,900.1

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