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Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Dec. 31, 2017	[1]
Segment Reporting Information [Line Items]
Total revenues	$ 1,837.7	$ 1,646.4	[1]
Earnings before income taxes	242.3	199.6	[1]
Identifiable assets at March 31, 2018 and 2017	16,127.5			$ 14,909.7
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues	1,195.6	1,084.9
Earnings before income taxes	318.9	276.5
Identifiable assets at March 31, 2018 and 2017	13,784.2	12,832.8
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues	229.9	209.9
Earnings before income taxes	21.6	22.1
Identifiable assets at March 31, 2018 and 2017	735.9	697.7
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	412.2	351.6
Earnings before income taxes	(98.2)	(99.0)
Identifiable assets at March 31, 2018 and 2017	$ 1,607.4	$ 1,659.4
[1] See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to ASC 606.

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