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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2021 | Dec. 31, 2020
Goodwill [Line Items] |  |
Beginning Balance | $ 6,127.0 | $ 5,618.5
Goodwill acquired during the year | 2,561.5 | 359.6
Goodwill adjustments related to appraisals and other acquisition adjustments | 50.8 | 31.5
Goodwill written-off related to sales of business | (1.0) |
Foreign currency translation adjustments during the year | (72.1) | 117.4
Ending Balance | 8,666.2 | 6,127.0
Brokerage [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 6,053.6 | 5,548.9
Goodwill acquired during the year | 2,511.8 | 359.6
Goodwill adjustments related to appraisals and other acquisition adjustments | 50.8 | 29.8
Goodwill written-off related to sales of business | (1.0) |
Foreign currency translation adjustments during the year | (70.6) | 115.3
Ending Balance | 8,544.6 | 6,053.6
Risk Management [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 70.5 | 66.6
Goodwill acquired during the year | 31.6 |
Goodwill adjustments related to appraisals and other acquisition adjustments |  | 1.7
Foreign currency translation adjustments during the year | (1.2) | 2.2
Ending Balance | 100.9 | 70.5
Corporate [Member] |  |
Goodwill [Line Items] |  |
Beginning Balance | 2.9 | 3.0
Goodwill acquired during the year | 18.1 |
Foreign currency translation adjustments during the year | (0.3) | (0.1)
Ending Balance | $ 20.7 | $ 2.9

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