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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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total revenues	$ 1,849.1	$ 1,825.2	$ 5,300.0	$ 5,473.6
Earnings before income taxes	209.5	114.8	737.2	542.1
Identifiable assets at September 30, 2020 and 2019	21,814.9	19,173.8	21,814.9	19,173.8	$ 19,634.8
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues	1,294.6	1,195.9	3,931.3	3,709.0
Earnings before income taxes	276.4	199.9	935.0	794.6
Identifiable assets at September 30, 2020 and 2019	18,641.2	16,261.6	18,641.2	16,261.6
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues	244.3	246.3	717.0	724.8
Earnings before income taxes	25.3	21.7	64.1	64.7
Identifiable assets at September 30, 2020 and 2019	964.5	920.8	964.5	920.8
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	310.2	383.0	651.7	1,039.8
Earnings before income taxes	(92.2)	(106.8)	(261.9)	(317.2)
Identifiable assets at September 30, 2020 and 2019	$ 2,209.2	$ 1,991.4	$ 2,209.2	$ 1,991.4

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