Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) $ in Millions	6 Months Ended
Jun. 30, 2019 USD ($)
Goodwill [Line Items]
Beginning Balance	$ 4,625.6
Goodwill acquired during the period	604.3
Goodwill adjustments due to appraisals and other acquisition adjustments	17.0
Goodwill written-off related to sales of business	(7.2)
Foreign currency translation adjustments during the period	2.2
Ending Balance	5,241.9
Brokerage [Member]
Goodwill [Line Items]
Beginning Balance	4,573.6
Goodwill acquired during the period	603.9
Goodwill adjustments due to appraisals and other acquisition adjustments	17.3
Goodwill written-off related to sales of business	(7.2)
Foreign currency translation adjustments during the period	2.4
Ending Balance	5,190.0
Risk Management [Member]
Goodwill [Line Items]
Beginning Balance	49.3
Goodwill adjustments due to appraisals and other acquisition adjustments	(0.3)
Foreign currency translation adjustments during the period	(0.2)
Ending Balance	48.8
Corporate [Member]
Goodwill [Line Items]
Beginning Balance	2.7
Goodwill acquired during the period	0.4
Ending Balance	$ 3.1

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.

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