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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) $ in Millions | 9 Months Ended
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Sep. 30, 2018 USD ($)
Goodwill [Line Items] |  |
Beginning Balance | $ 4,164.8 | [1]
Goodwill acquired during the period | 404.7 |
Goodwill adjustments due to appraisals and other acquisition adjustments | 1.4 |
Foreign currency translation adjustments during the period | (69.6) |
Ending Balance | 4,501.3 |
As Previously Reported [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 4,197.9 |
Accounting Standards Update 2014-09 [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | (33.1) |
Brokerage [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 4,119.2 |
Goodwill acquired during the period | 397.4 |
Goodwill adjustments due to appraisals and other acquisition adjustments | 3.7 |
Foreign currency translation adjustments during the period | (68.3) |
Ending Balance | 4,452.0 |
Brokerage [Member] | As Previously Reported [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 4,152.3 |
Brokerage [Member] | Accounting Standards Update 2014-09 [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | (33.1) |
Risk Management [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 42.6 |
Goodwill acquired during the period | 7.3 |
Goodwill adjustments due to appraisals and other acquisition adjustments | (2.3) |
Foreign currency translation adjustments during the period | (0.9) |
Ending Balance | 46.7 |
Risk Management [Member] | As Previously Reported [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 42.6 |
Corporate [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 3.0 |
Foreign currency translation adjustments during the period | (0.4) |
Ending Balance | 2.6 |
Corporate [Member] | As Previously Reported [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | $ 3.0 |
[1] | See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to Topic 606.

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