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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Goodwill [Line Items] |  |
Beginning Balance | $ 3,767.8 | $ 3,662.9
Goodwill acquired during the year | 304.5 | 225.4
Goodwill adjustments related to appraisals and other acquisition adjustments | 14.7 | 3.4
Foreign currency translation adjustments during the year | 110.9 | (123.9)
Ending Balance | 4,197.9 | 3,767.8
Brokerage [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 3,736.9 | 3,635.6
Goodwill acquired during the year | 290.4 | 222.6
Goodwill adjustments related to appraisals and other acquisition adjustments | 14.7 | 1.8
Foreign currency translation adjustments during the year | 110.3 | (123.1)
Ending Balance | 4,152.3 | 3,736.9
Risk Management [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 28.1 | 27.3
Goodwill acquired during the year | 14.1 |
Goodwill adjustments related to appraisals and other acquisition adjustments |  | 1.6
Foreign currency translation adjustments during the year | 0.4 | (0.8)
Ending Balance | 42.6 | 28.1
Corporate [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 2.8 |
Goodwill acquired during the year |  | 2.8
Foreign currency translation adjustments during the year | 0.2 |
Ending Balance | $ 3.0 | $ 2.8

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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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- 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 Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 + Details
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