Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) $ in Millions	3 Months Ended
Mar. 31, 2021 USD ($)
Goodwill [Line Items]
Beginning Balance	$ 6,127.0
Goodwill acquired during the period	288.5
Goodwill adjustments due to appraisals and other acquisition adjustments	52.1
Goodwill written-off related to sales of business and impairment	(1.0)
Foreign currency translation adjustments during the period	44.3
Ending Balance	6,510.9
Brokerage [Member]
Goodwill [Line Items]
Beginning Balance	6,053.6
Goodwill acquired during the period	269.5
Goodwill adjustments due to appraisals and other acquisition adjustments	52.1
Goodwill written-off related to sales of business and impairment	(1.0)
Foreign currency translation adjustments during the period	43.9
Ending Balance	6,418.1
Risk Management [Member]
Goodwill [Line Items]
Beginning Balance	70.5
Goodwill acquired during the period	19.0
Foreign currency translation adjustments during the period	0.4
Ending Balance	89.9
Corporate [Member]
Goodwill [Line Items]
Beginning Balance	2.9
Ending Balance	$ 2.9

Goodwill written off related to sale of business and impairment.

No definition available.

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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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

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.

+ References

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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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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Amount of increase (decrease) from foreign currency translation adjustments and purchase accounting adjustments 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://fasb.org/us-gaap/role/ref/legacyRef
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