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Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Segment Reporting Information [Line Items]
Total revenues	$ 2,435.3	$ 2,165.0
Earnings before income taxes	537.7	408.4
Identifiable assets at March 31, 2022 and 2021	43,161.5	24,414.5	$ 33,345.0
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues	2,122.6	1,610.2
Earnings before income taxes	618.4	480.3
Identifiable assets at March 31, 2022 and 2021	39,590.1	21,242.3
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues	289.9	252.7
Earnings before income taxes	32.3	24.1
Identifiable assets at March 31, 2022 and 2021	1,047.4	1,028.1
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	22.8	302.1
Earnings before income taxes	(113.0)	(96.0)
Identifiable assets at March 31, 2022 and 2021	$ 2,524.0	$ 2,144.1

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