Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Goodwill [Line Items]
Beginning Balance	$ 1,472.7	$ 1,155.3
Goodwill acquired during the year	665.0	308.8
Goodwill related to earnouts recognized during the year		0.1
Goodwill adjustments related to appraisals and other acquisition adjustments	3.3	(0.8)
Goodwill related to transfers of operations between segments
Foreign currency translation adjustments during the year	4.2	9.3
Ending Balance	2,145.2	1,472.7
Brokerage [Member]
Goodwill [Line Items]
Beginning Balance	1,451.4	1,136.6
Goodwill acquired during the year	664.1	308.1
Goodwill related to earnouts recognized during the year		0.1
Goodwill adjustments related to appraisals and other acquisition adjustments	3.3	(0.6)
Goodwill related to transfers of operations between segments		(2.0)
Foreign currency translation adjustments during the year	4.1	9.2
Ending Balance	2,122.9	1,451.4
Risk Management [Member]
Goodwill [Line Items]
Beginning Balance	21.3	18.7
Goodwill acquired during the year	0.9	0.7
Goodwill adjustments related to appraisals and other acquisition adjustments		(0.2)
Goodwill related to transfers of operations between segments		2.0
Foreign currency translation adjustments during the year	0.1	0.1
Ending Balance	22.3	21.3
Corporate [Member]
Goodwill [Line Items]
Beginning Balance
Goodwill acquired during the year
Goodwill related to earnouts recognized during the year
Goodwill adjustments related to appraisals and other acquisition adjustments
Goodwill related to transfers of operations between segments
Foreign currency translation adjustments during the year
Ending Balance