Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Total revenues	$ 1,825.2	$ 1,778.5	$ 5,473.6	$ 5,276.6
Earnings before income taxes	114.8	90.3	542.1	436.2
Identifiable assets at September 30, 2019 and 2018	19,173.8	16,114.7	19,173.8	16,114.7	$ 16,334.0
Operating Segments [Member] \| Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues	1,195.9	1,049.4	3,709.0	3,245.1
Earnings before income taxes	199.9	163.2	794.6	652.7
Identifiable assets at September 30, 2019 and 2018	16,261.6	13,700.7	16,261.6	13,700.7
Operating Segments [Member] \| Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues	246.3	236.4	724.8	703.6
Earnings before income taxes	21.7	22.3	64.7	67.9
Identifiable assets at September 30, 2019 and 2018	920.8	748.3	920.8	748.3
Operating Segments [Member] \| Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	383.0	492.7	1,039.8	1,327.9
Earnings before income taxes	(106.8)	(95.2)	(317.2)	(284.4)
Identifiable assets at September 30, 2019 and 2018	$ 1,991.4	$ 1,665.7	$ 1,991.4	$ 1,665.7

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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