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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions | Mar. 31, 2026 | Dec. 31, 2025 | Mar. 31, 2025
--- | --- | --- | ---
Goodwill [Line Items] |  |  |
Total goodwill | $ 22,958 | $ 22,593 | $ 12,715
United States |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 15,777 | 16,537 |
United Kingdom |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 3,607 | 3,031 |
Canada |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 624 | 628 |
Australia |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 879 | 829 |
New Zealand |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 234 | 233 |
Other foreign |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 1,837 | 1,335 |
Brokerage |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 22,424 | 22,078 |
Risk Management |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 517 | 497 |
Corporate |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 17 | 18 |
Operating Segments | Brokerage |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 22,424 | 22,078 | 12,277
Operating Segments | Brokerage | United States |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 15,668 | 16,428 |
Operating Segments | Brokerage | United Kingdom |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 3,468 | 2,889 |
Operating Segments | Brokerage | Canada |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 624 | 628 |
Operating Segments | Brokerage | Australia |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 632 | 591 |
Operating Segments | Brokerage | New Zealand |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 225 | 225 |
Operating Segments | Brokerage | Other foreign |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 1,807 | 1,317 |
Operating Segments | Risk Management |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 517 | 497 | 419
Operating Segments | Risk Management | United States |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 109 | 109 |
Operating Segments | Risk Management | United Kingdom |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 139 | 142 |
Operating Segments | Risk Management | Canada |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 0 | 0 |
Operating Segments | Risk Management | Australia |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 247 | 238 |
Operating Segments | Risk Management | New Zealand |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 9 | 8 |
Operating Segments | Risk Management | Other foreign |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 13 | 0 |
Operating Segments | Corporate |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 17 | 18 | $ 19
Operating Segments | Corporate | United States |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 0 | 0 |
Operating Segments | Corporate | United Kingdom |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 0 | 0 |
Operating Segments | Corporate | Canada |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 0 | 0 |
Operating Segments | Corporate | Australia |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 0 | 0 |
Operating Segments | Corporate | New Zealand |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | 0 | 0 |
Operating Segments | Corporate | Other foreign |  |  |
Goodwill [Line Items] |  |  |
Total goodwill | $ 17 | $ 18 |

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