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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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total revenues	$ 1,584.5	$ 1,482.3	$ 4,560.6	$ 4,209.8
Earnings before income taxes	96.7	94.9	296.0	288.1
Identifiable assets at September 30, 2017 and 2016	12,810.4		12,810.4		$ 11,489.6
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues	953.7	877.6	2,830.3	2,642.2
Earnings before income taxes	161.6	149.1	491.8	427.3
Identifiable assets at September 30, 2017 and 2016	10,275.9	8,972.7	10,275.9	8,972.7
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues	200.2	176.7	571.5	532.5
Earnings before income taxes	27.8	21.5	73.6	67.9
Identifiable assets at September 30, 2017 and 2016	733.0	679.6	733.0	679.6
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	430.6	428.0	1,158.8	1,035.1
Earnings before income taxes	(92.7)	(75.7)	(269.4)	(207.1)
Identifiable assets at September 30, 2017 and 2016	$ 1,801.5	$ 1,548.0	$ 1,801.5	$ 1,548.0

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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