Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) $ in Millions	9 Months Ended
Sep. 30, 2021 USD ($)
Goodwill [Line Items]
Beginning Balance	$ 6,127.0
Goodwill acquired during the period	453.5
Goodwill adjustments due to appraisals and other acquisition adjustments	50.5
Goodwill written-off related to sales of business and impairment	(1.0)
Foreign currency translation adjustments during the period	(2.5)
Ending Balance	6,627.5
Brokerage [Member]
Goodwill [Line Items]
Beginning Balance	6,053.6
Goodwill acquired during the period	421.9
Goodwill adjustments due to appraisals and other acquisition adjustments	50.5
Goodwill written-off related to sales of business and impairment	(1.0)
Foreign currency translation adjustments during the period	(2.2)
Ending Balance	6,522.8
Risk Management [Member]
Goodwill [Line Items]
Beginning Balance	70.5
Goodwill acquired during the period	31.6
Foreign currency translation adjustments during the period	(0.3)
Ending Balance	101.8
Corporate [Member]
Goodwill [Line Items]
Beginning Balance	2.9
Ending Balance	$ 2.9

Goodwill written off related to sale of business and impairment.

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- 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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Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

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Amount of increase (decrease) from adjustments after acquisition date under purchase accounting of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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