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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions | Sep. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Goodwill [Line Items] |  |
Total goodwill | $ 5,314.8 | $ 4,625.6
Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 2,983.0 | 2,744.9
United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 1,150.9 | 762.9
Canada [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 445.4 | 378.6
Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 414.3 | 406.6
New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 206.0 | 219.8
Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 115.2 | 112.8
Brokerage [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 5,246.8 | 4,573.6
Brokerage [Member] | Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 2,950.0 | 2,715.3
Brokerage [Member] | United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 1,138.6 | 753.7
Brokerage [Member] | Canada [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 445.4 | 378.6
Brokerage [Member] | Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 404.2 | 406.3
Brokerage [Member] | New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 196.5 | 209.6
Brokerage [Member] | Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 112.1 | 110.1
Risk Management [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 64.9 | 49.3
Risk Management [Member] | Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 33.0 | 29.6
Risk Management [Member] | United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 12.3 | 9.2
Risk Management [Member] | Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 10.1 | 0.3
Risk Management [Member] | New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 9.5 | 10.2
Corporate [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 3.1 | 2.7
Corporate [Member] | Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | $ 3.1 | $ 2.7

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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://fasb.org/us-gaap/role/ref/legacyRef -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 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266 + Details
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