Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Line Items]
Beginning Balance	$ 3,662.9	$ 3,449.6
Goodwill acquired during the year	225.4	354.6
Goodwill adjustments related to appraisals and other acquisition adjustments	3.4	25.3
Foreign currency translation adjustments during the year	(123.9)	(166.6)
Ending Balance	3,767.8	3,662.9
Brokerage [Member]
Goodwill [Line Items]
Beginning Balance	3,635.6	3,427.5
Goodwill acquired during the year	222.6	352.6
Goodwill related to transfers of operations between segments		(3.4)
Goodwill adjustments related to appraisals and other acquisition adjustments	1.8	25.3
Foreign currency translation adjustments during the year	(123.1)	(166.4)
Ending Balance	3,736.9	3,635.6
Risk Management Services Management Segment [Member]
Goodwill [Line Items]
Beginning Balance	27.3	22.1
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	1.6
Foreign currency translation adjustments during the year	(0.8)	(0.2)
Ending Balance	28.1	$ 27.3
Corporate [Member]
Goodwill [Line Items]
Goodwill acquired during the year	2.8
Ending Balance	$ 2.8