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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2015
Goodwill [Line Items]
Beginning Balance	$ 4,164.8	[1]	$ 3,753.2
Goodwill acquired during the year	584.6		304.5
Goodwill adjustments related to appraisals and other acquisition adjustments	(0.1)		14.7
Foreign currency translation adjustments during the year	(123.7)		110.9
Ending Balance	4,625.6		4,164.8	[1]
As reported [Member]
Goodwill [Line Items]
Beginning Balance			3,767.8
Accounting Standards Update 2014-09 [Member]
Goodwill [Line Items]
Adoption of Topic 606 related to 2016 acquisitions					$ (127.5)
Accounting Standards Update 2014-09 [Member] | 2016 Acquisitions [Member]
Goodwill [Line Items]
Adoption of Topic 606 related to 2016 acquisitions			(14.6)
Accounting Standards Update 2014-09 [Member] | 2017 Acquisitions [Member]
Goodwill [Line Items]
Adoption of Topic 606 related to 2016 acquisitions			(18.5)
Brokerage [Member]
Goodwill [Line Items]
Beginning Balance	4,119.2		3,722.3
Goodwill acquired during the year	574.7		290.4
Goodwill adjustments related to appraisals and other acquisition adjustments	2.2		14.7
Foreign currency translation adjustments during the year	(122.5)		110.3
Ending Balance	4,573.6		4,119.2
Brokerage [Member] | As reported [Member]
Goodwill [Line Items]
Beginning Balance			3,736.9
Brokerage [Member] | Accounting Standards Update 2014-09 [Member] | 2016 Acquisitions [Member]
Goodwill [Line Items]
Adoption of Topic 606 related to 2016 acquisitions			(14.6)
Brokerage [Member] | Accounting Standards Update 2014-09 [Member] | 2017 Acquisitions [Member]
Goodwill [Line Items]
Adoption of Topic 606 related to 2016 acquisitions			(18.5)
Risk Management [Member]
Goodwill [Line Items]
Beginning Balance	42.6		28.1
Goodwill acquired during the year	9.9		14.1
Goodwill adjustments related to appraisals and other acquisition adjustments	(2.3)
Foreign currency translation adjustments during the year	(0.9)		0.4
Ending Balance	49.3		42.6
Risk Management [Member] | As reported [Member]
Goodwill [Line Items]
Beginning Balance			28.1
Corporate [Member]
Goodwill [Line Items]
Beginning Balance	3.0		2.8
Foreign currency translation adjustments during the year	(0.3)		0.2
Ending Balance	$ 2.7		3.0
Corporate [Member] | As reported [Member]
Goodwill [Line Items]
Beginning Balance			$ 2.8
[1] See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to Topic 606.

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