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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2015
Goodwill [Line Items] |  |  |  |  |
Beginning Balance | $ 4,164.8 | [1] | $ 3,753.2 |  |
Goodwill acquired during the year | 584.6 |  | 304.5 |  |
Goodwill adjustments related to appraisals and other acquisition adjustments | (0.1) |  | 14.7 |  |
Foreign currency translation adjustments during the year | (123.7) |  | 110.9 |  |
Ending Balance | 4,625.6 |  | 4,164.8 | [1] |
As reported [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Beginning Balance |  |  | 3,767.8 |  |
Accounting Standards Update 2014-09 [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Adoption of Topic 606 related to 2016 acquisitions |  |  |  |  | $ (127.5)
Accounting Standards Update 2014-09 [Member] | 2016 Acquisitions [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Adoption of Topic 606 related to 2016 acquisitions |  |  | (14.6) |  |
Accounting Standards Update 2014-09 [Member] | 2017 Acquisitions [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Adoption of Topic 606 related to 2016 acquisitions |  |  | (18.5) |  |
Brokerage [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Beginning Balance | 4,119.2 |  | 3,722.3 |  |
Goodwill acquired during the year | 574.7 |  | 290.4 |  |
Goodwill adjustments related to appraisals and other acquisition adjustments | 2.2 |  | 14.7 |  |
Foreign currency translation adjustments during the year | (122.5) |  | 110.3 |  |
Ending Balance | 4,573.6 |  | 4,119.2 |  |
Brokerage [Member] | As reported [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Beginning Balance |  |  | 3,736.9 |  |
Brokerage [Member] | Accounting Standards Update 2014-09 [Member] | 2016 Acquisitions [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Adoption of Topic 606 related to 2016 acquisitions |  |  | (14.6) |  |
Brokerage [Member] | Accounting Standards Update 2014-09 [Member] | 2017 Acquisitions [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Adoption of Topic 606 related to 2016 acquisitions |  |  | (18.5) |  |
Risk Management [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Beginning Balance | 42.6 |  | 28.1 |  |
Goodwill acquired during the year | 9.9 |  | 14.1 |  |
Goodwill adjustments related to appraisals and other acquisition adjustments | (2.3) |  |  |  |
Foreign currency translation adjustments during the year | (0.9) |  | 0.4 |  |
Ending Balance | 49.3 |  | 42.6 |  |
Risk Management [Member] | As reported [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Beginning Balance |  |  | 28.1 |  |
Corporate [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Beginning Balance | 3.0 |  | 2.8 |  |
Foreign currency translation adjustments during the year | (0.3) |  | 0.2 |  |
Ending Balance | $ 2.7 |  | 3.0 |  |
Corporate [Member] | As reported [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Beginning Balance |  |  | $ 2.8 |  |
[1] | See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to Topic 606.

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