v3.26.3
Revenue - Schedule of Disaggregation of Revenue (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax $ 2,404 $ 2,240 $ 7,195 $ 6,635
Reimbursable expenses and other [1] 21 14 72 58
Interest and other income 56 48 143 137
Total revenue 2,460 2,288 7,338 6,772
HWC [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 1,362 1,269 3,921 3,632
Reimbursable expenses and other [1] 21 16 62 51
Interest and other income 9 8 26 25
Total revenue 1,371 1,277 3,947 3,657
R&B [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 1,047 976 3,274 3,004
Reimbursable expenses and other [1] 5 3 12 9
Interest and other income 45 34 105 86
Total revenue 1,092 1,010 3,379 3,090
Corporate [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax [1] (5) (5) 0 (1)
Reimbursable expenses and other [1] (5) (5) (2) (2)
Interest and other income [1] 2 6 12 26
Total revenue [1] (3) 1 12 25
Broking [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 1,000 915 3,140 2,847
Broking [Member] | HWC [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 143 121 488 417
Broking [Member] | R&B [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 857 794 2,652 2,430
Broking [Member] | Corporate [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 854 818 2,569 2,420
Consulting [Member] | HWC [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 750 714 2,226 2,097
Consulting [Member] | R&B [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 104 104 341 322
Consulting [Member] | Corporate [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax [1] 0 0 2 1
Outsourced administration [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 299 299 901 845
Outsourced administration [Member] | HWC [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 285 282 851 790
Outsourced administration [Member] | R&B [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 14 17 50 55
Outsourced administration [Member] | Corporate [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 230 194 513 465
Other [Member] | HWC [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 163 136 294 277
Other [Member] | R&B [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 67 58 219 188
Other [Member] | Corporate [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 2,383 2,226 7,123 6,577
Service [Member] | HWC [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 1,341 1,253 3,859 3,581
Service [Member] | R&B [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 1,042 973 3,262 2,995
Service [Member] | Corporate [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax [1] $ 0 $ 0 $ 2 $ 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.