Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Goodwill [Line Items]
Beginning Balance	$ 3,449.6	$ 2,145.2
Goodwill acquired during the year	354.6	1,448.3
Goodwill related to transfers of operations between segments	0.0
Goodwill adjustments related to appraisals and other acquisition adjustments	25.3	(8.8)
Goodwill written-off related to sales of business		(0.6)
Foreign currency translation adjustments during the year	(166.6)	(134.5)
Ending Balance	3,662.9	3,449.6
Brokerage [Member]
Goodwill [Line Items]
Beginning Balance	3,427.5	2,122.9
Goodwill acquired during the year	352.6	1,448.3
Goodwill related to transfers of operations between segments	(3.4)
Goodwill adjustments related to appraisals and other acquisition adjustments	25.3	(8.8)
Goodwill written-off related to sales of business		(0.6)
Foreign currency translation adjustments during the year	(166.4)	(134.3)
Ending Balance	3,635.6	3,427.5
Risk Management [Member]
Goodwill [Line Items]
Beginning Balance	22.1	22.3
Goodwill acquired during the year	2.0
Goodwill related to transfers of operations between segments	3.4
Goodwill adjustments related to appraisals and other acquisition adjustments	0.0
Foreign currency translation adjustments during the year	(0.2)	(0.2)
Ending Balance	27.3	22.1
Corporate [Member]
Goodwill [Line Items]
Beginning Balance	0.0
Goodwill acquired during the year	0.0
Goodwill related to transfers of operations between segments	0.0
Goodwill adjustments related to appraisals and other acquisition adjustments	0.0
Foreign currency translation adjustments during the year	0.0
Ending Balance	$ 0.0	$ 0.0