Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total revenues	$ 1,482.3	$ 1,454.8	$ 4,209.8	$ 4,057.5
Earnings before income taxes	94.9	105.7	288.1	265.1
Identifiable assets at September 30, 2016 and 2015	11,200.3		11,200.3		$ 10,910.5
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues	877.6	835.5	2,642.2	2,472.2
Earnings before income taxes	149.1	122.5	427.3	340.1
Identifiable assets at September 30, 2016 and 2015	8,972.7	8,638.2	8,972.7	8,638.2
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues	176.7	179.2	532.5	545.8
Earnings before income taxes	21.5	22.4	67.9	72.7
Identifiable assets at September 30, 2016 and 2015	679.6	640.4	679.6	640.4
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	428.0	440.1	1,035.1	1,039.5
Earnings before income taxes	(75.7)	(39.2)	(207.1)	(147.7)
Identifiable assets at September 30, 2016 and 2015	$ 1,548.0	$ 1,231.8	$ 1,548.0	$ 1,231.8