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Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Total revenues	$ 2,165.0	$ 1,866.9
Earnings before income taxes	408.4	356.0
Identifiable assets at March 31, 2021 and 2020	24,414.5	20,836.5	$ 22,331.4
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues	1,610.2	1,435.6
Earnings before income taxes	480.3	410.8
Identifiable assets at March 31, 2021 and 2020	21,242.3	18,115.1
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues	252.7	249.5
Earnings before income taxes	24.1	25.6
Identifiable assets at March 31, 2021 and 2020	1,028.1	909.8
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	302.1	181.8
Earnings before income taxes	(96.0)	(80.4)
Identifiable assets at March 31, 2021 and 2020	$ 2,144.1	$ 1,811.6

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