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Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total revenues	$ 1,412.7	$ 1,300.4
Earnings before income taxes	52.3	55.7
Identifiable assets at March 31, 2017 and 2016	11,780.6		$ 11,489.6
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues	878.5	825.5
Earnings before income taxes	117.7	100.0
Identifiable assets at March 31, 2017 and 2016	9,422.6	8,681.9
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues	182.6	179.3
Earnings before income taxes	22.9	24.0
Identifiable assets at March 31, 2017 and 2016	691.1	690.4
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	351.6	295.6
Earnings before income taxes	(88.3)	(68.3)
Identifiable assets at March 31, 2017 and 2016	$ 1,666.9	$ 1,340.3

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 210
-Section S99
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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