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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018
Goodwill [Line Items] |  |
Beginning Balance | $ 4,625.6 | $ 4,164.8
Goodwill acquired during the year | 975.7 | 584.6
Goodwill adjustments related to appraisals and other acquisition adjustments |  | (0.1)
Goodwill written-off related to sales of business | (7.2) |
Foreign currency translation adjustments during the year | 24.4 | (123.7)
Ending Balance | 5,618.5 | 4,625.6
Brokerage [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 4,573.6 | 4,119.2
Goodwill acquired during the year | 958.4 | 574.7
Goodwill adjustments related to appraisals and other acquisition adjustments | 0.2 | 2.2
Goodwill written-off related to sales of business | (7.2) |
Foreign currency translation adjustments during the year | 23.9 | (122.5)
Ending Balance | 5,548.9 | 4,573.6
Risk Management [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 49.3 | 42.6
Goodwill acquired during the year | 16.9 | 9.9
Goodwill adjustments related to appraisals and other acquisition adjustments | (0.2) | (2.3)
Foreign currency translation adjustments during the year | 0.6 | (0.9)
Ending Balance | 66.6 | 49.3
Corporate [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 2.7 | 3.0
Goodwill acquired during the year | 0.4 |
Foreign currency translation adjustments during the year | (0.1) | (0.3)
Ending Balance | $ 3.0 | $ 2.7

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