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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions | Mar. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Goodwill [Line Items] |  |
Total goodwill | $ 11,532.1 | $ 11,475.6
Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 6,500.0 | 6,379.3
United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 2,511.7 | 2,511.9
Canada [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 607.8 | 623.7
Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 623.7 | 650.5
New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 202.5 | 213.8
Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 1,086.4 | 1,096.4
Brokerage [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 11,281.0 | 11,217.8
Risk Management [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 232.1 | 238.8
Corporate [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 19.0 | 19.0
Operating Segments [Member] | Brokerage [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 11,281.0 | 11,217.8
Operating Segments [Member] | Brokerage [Member] | Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 6,425.2 | 6,304.5
Operating Segments [Member] | Brokerage [Member] | United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 2,493.3 | 2,493.4
Operating Segments [Member] | Brokerage [Member] | Canada [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 607.8 | 623.7
Operating Segments [Member] | Brokerage [Member] | Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 493.8 | 514.6
Operating Segments [Member] | Brokerage [Member] | New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 193.5 | 204.2
Operating Segments [Member] | Brokerage [Member] | Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 1,067.4 | 1,077.4
Operating Segments [Member] | Risk Management [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 232.1 | 238.8
Operating Segments [Member] | Risk Management [Member] | Unites States [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 74.8 | 74.8
Operating Segments [Member] | Risk Management [Member] | United Kingdom [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 18.4 | 18.5
Operating Segments [Member] | Risk Management [Member] | Australia [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 129.9 | 135.9
Operating Segments [Member] | Risk Management [Member] | New Zealand [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 9.0 | 9.6
Operating Segments [Member] | Corporate [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | 19.0 | 19.0
Operating Segments [Member] | Corporate [Member] | Other Foreign [Member] |  |
Goodwill [Line Items] |  |
Total goodwill | $ 19.0 | $ 19.0

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