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Revenue - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax |  | $ 2,240 | $ 2,238 | $ 6,635 | $ 6,752
Reimbursable expenses and other | [1] | 14 | 15 | 58 | 55
Interest and other income |  | 48 | 51 | 137 | 143
Total revenue |  | 2,288 | 2,289 | 6,772 | 6,895
HWC [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax |  | 1,269 | 1,334 | 3,632 | 3,944
Reimbursable expenses and other | [1] | 16 | 17 | 51 | 49
Interest and other income |  | 8 | 11 | 25 | 29
Total revenue |  | 1,277 | 1,345 | 3,657 | 3,973
R&B [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax |  | 976 | 909 | 3,004 | 2,810
Reimbursable expenses and other | [1] | 3 | 3 | 9 | 9
Interest and other income |  | 34 | 34 | 86 | 96
Total revenue |  | 1,010 | 943 | 3,090 | 2,906
Corporate Segment [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | [1] | (5) | (5) | (1) | (2)
Reimbursable expenses and other | [1] | (5) | (5) | (2) | (3)
Interest and other income | [1] | 6 | 6 | 26 | 18
Total revenue | [1] | 1 | 1 | 25 | 16
Broking [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax |  | 915 | 1,006 | 2,847 | 3,140
Broking [Member] | HWC [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax |  | 121 | 262 | 417 | 876
Broking [Member] | R&B [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax |  | 794 | 744 | 2,430 | 2,264
Broking [Member] | Corporate Segment [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | [1] | 0 | 0 | 0 | 0
Consulting [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax |  | 818 | 746 | 2,420 | 2,261
Consulting [Member] | HWC [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax |  | 714 | 653 | 2,097 | 1,966
Consulting [Member] | R&B [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax |  | 104 | 93 | 322 | 294
Consulting [Member] | Corporate Segment [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | [1] | 0 | 0 | 1 | 1
Outsourced administration [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax |  | 299 | 284 | 845 | 854
Outsourced administration [Member] | HWC [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax |  | 282 | 268 | 790 | 791
Outsourced administration [Member] | R&B [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax |  | 17 | 16 | 55 | 63
Outsourced administration [Member] | Corporate Segment [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | [1] | 0 | 0 | 0 | 0
Other [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax |  | 194 | 187 | 465 | 442
Other [Member] | HWC [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax |  | 136 | 134 | 277 | 262
Other [Member] | R&B [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax |  | 58 | 53 | 188 | 180
Other [Member] | Corporate Segment [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | [1] | 0 | 0 | 0 | 0
Service [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax |  | 2,226 | 2,223 | 6,577 | 6,697
Service [Member] | HWC [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax |  | 1,253 | 1,317 | 3,581 | 3,895
Service [Member] | R&B [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax |  | 973 | 906 | 2,995 | 2,801
Service [Member] | Corporate Segment [Member] |  |  |  |  |
Disaggregation Of Revenue [Line Items] |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | [1] | $ 0 | $ 0 | $ 1 | $ 1
[1] | Reimbursable expenses and other, as well as Corporate revenue, are excluded from segment revenue, but included in total revenue on the condensed consolidated statements of comprehensive income. Amounts included in Corporate revenue may include eliminations, adjustments to reserves and impacts from hedged revenue transactions.

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