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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($)
Goodwill [Line Items]
Beginning Balance	$ 5,618.5
Goodwill acquired during the period	200.7
Goodwill adjustments due to appraisals and other acquisition adjustments	30.5
Foreign currency translation adjustments during the period	(17.5)
Ending Balance	5,832.2
Brokerage [Member]
Goodwill [Line Items]
Beginning Balance	5,548.9
Goodwill acquired during the period	200.7
Goodwill adjustments due to appraisals and other acquisition adjustments	29.2
Foreign currency translation adjustments during the period	(17.4)
Ending Balance	5,761.4
Risk Management [Member]
Goodwill [Line Items]
Beginning Balance	66.6
Goodwill adjustments due to appraisals and other acquisition adjustments	1.3
Ending Balance	67.9
Corporate [Member]
Goodwill [Line Items]
Beginning Balance	3.0
Foreign currency translation adjustments during the period	(0.1)
Ending Balance	$ 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

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