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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.