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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) $ in Millions | 9 Months Ended
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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

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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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975 + Details
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