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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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Total revenues	$ 1,657.8	$ 1,660.4	$ 3,648.4	$ 3,498.1
Earnings (loss) before income taxes	105.5	103.6	427.3	345.9
Identifiable assets at June 30, 2019 and 2018	19,200.3		19,200.3		$ 16,334.0
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues	1,131.2	1,000.1	2,513.1	2,195.7
Earnings (loss) before income taxes	182.3	170.6	594.7	489.5
Identifiable assets at June 30, 2019 and 2018	16,464.3	13,779.0	16,464.3	13,779.0
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues	242.1	237.3	478.5	467.2
Earnings (loss) before income taxes	21.0	24.0	43.0	45.6
Identifiable assets at June 30, 2019 and 2018	847.9	739.5	847.9	739.5
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	284.5	423.0	656.8	835.2
Earnings (loss) before income taxes	(97.8)	(91.0)	(210.4)	(189.2)
Identifiable assets at June 30, 2019 and 2018	$ 1,888.1	$ 1,712.0	$ 1,888.1	$ 1,712.0

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