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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, 2018	Sep. 30, 2017		Sep. 30, 2018	Sep. 30, 2017		Dec. 31, 2017	[1]
Segment Reporting Information [Line Items]
Total revenues	$ 1,778.5	$ 1,593.7	[1]	$ 5,276.6	$ 4,729.6	[1]
Earnings before income taxes	90.3	76.9	[1]	436.2	330.6	[1]
Identifiable assets at September 30, 2018 and 2017	16,114.7			16,114.7			$ 14,909.7
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues	1,049.4	938.7		3,245.1	2,919.7
Earnings before income taxes	163.2	141.2		652.7	542.8
Identifiable assets at September 30, 2018 and 2017	13,700.7	13,019.7		13,700.7	13,019.7
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues	236.4	224.4		703.6	651.1
Earnings before income taxes	22.3	26.5		67.9	69.2
Identifiable assets at September 30, 2018 and 2017	748.3	738.0		748.3	738.0
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	492.7	430.6		1,327.9	1,158.8
Earnings before income taxes	(95.2)	(90.8)		(284.4)	(281.4)
Identifiable assets at September 30, 2018 and 2017	$ 1,665.7	$ 1,746.7		$ 1,665.7	$ 1,746.7
[1] See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to Topic 606.

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