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Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017		Jun. 30, 2018	Jun. 30, 2017		Dec. 31, 2017	[1]
Segment Reporting Information [Line Items]
Total revenues	$ 1,660.4	$ 1,489.5	[1]	$ 3,498.1	$ 3,135.9	[1]
Earnings before income taxes	103.6	54.1	[1]	345.9	253.7	[1]
Identifiable assets at June 30, 2018 and 2017	16,230.5			16,230.5			$ 14,909.7
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues	1,000.1	896.1		2,195.7	1,981.0
Earnings before income taxes	170.6	125.1		489.5	401.6
Identifiable assets at June 30, 2018 and 2017	13,779.0	13,202.9		13,779.0	13,202.9
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues	237.3	216.8		467.2	426.7
Earnings before income taxes	24.0	20.6		45.6	42.7
Identifiable assets at June 30, 2018 and 2017	739.5	714.7		739.5	714.7
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	423.0	376.6		835.2	728.2
Earnings before income taxes	(91.0)	(91.6)		(189.2)	(190.6)
Identifiable assets at June 30, 2018 and 2017	$ 1,712.0	$ 1,761.8		$ 1,712.0	$ 1,761.8
[1] See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to ASC 606.

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