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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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Total revenues	$ 2,138.2	$ 1,849.1	$ 6,236.7	$ 5,300.0
Earnings before income taxes	231.4	209.5	862.1	737.2
Identifiable assets at September 30, 2021 and 2020	26,915.8	21,814.9	26,915.8	21,814.9	$ 22,331.4
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues	1,499.7	1,294.6	4,500.1	3,931.3
Earnings before income taxes	334.5	276.4	1,114.5	935.0
Identifiable assets at September 30, 2021 and 2020	21,184.7	18,641.2	21,184.7	18,641.2
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues	280.6	244.3	815.0	717.0
Earnings before income taxes	29.5	25.3	87.0	64.1
Identifiable assets at September 30, 2021 and 2020	1,079.7	964.5	1,079.7	964.5
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	357.9	310.2	921.6	651.7
Earnings before income taxes	(132.6)	(92.2)	(339.4)	(261.9)
Identifiable assets at September 30, 2021 and 2020	$ 4,651.4	$ 2,209.2	$ 4,651.4	$ 2,209.2

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