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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($)
Goodwill [Line Items]
Beginning Balance	$ 3,767.8
Goodwill acquired during the period	228.1
Goodwill adjustments due to appraisals and other acquisition adjustments	14.4
Foreign currency translation adjustments during the period	150.1
Balance as of September 30, 2017	4,160.4
Brokerage [Member]
Goodwill [Line Items]
Beginning Balance	3,736.9
Goodwill acquired during the period	214.0
Goodwill adjustments due to appraisals and other acquisition adjustments	14.4
Foreign currency translation adjustments during the period	149.2
Balance as of September 30, 2017	4,114.5
Risk Management [Member]
Goodwill [Line Items]
Beginning Balance	28.1
Goodwill acquired during the period	14.1
Foreign currency translation adjustments during the period	0.8
Balance as of September 30, 2017	43.0
Corporate [Member]
Goodwill [Line Items]
Beginning Balance	2.8
Foreign currency translation adjustments during the period	0.1
Balance as of September 30, 2017	$ 2.9

X

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

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+ References

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+ References

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+ References

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