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Revenue - Schedule of Disaggregation of Revenue (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2025
Jun. 30, 2024
Jun. 30, 2025
Jun. 30, 2024
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax $ 2,215 $ 2,219 $ 4,395 $ 4,514
Reimbursable expenses and other [1] 23 20 44 40
Interest and other income 46 46 89 92
Total revenue 2,261 2,265 4,484 4,606
HWC [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 1,190 1,266 2,363 2,610
Reimbursable expenses and other [1] 18 15 35 32
Interest and other income 8 9 17 18
Total revenue 1,198 1,275 2,380 2,628
R&B [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 1,022 951 2,028 1,901
Reimbursable expenses and other [1] 3 3 6 6
Interest and other income 28 31 52 62
Total revenue 1,050 982 2,080 1,963
Corporate Segment [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax [1] 3 2 4 3
Reimbursable expenses and other [1] 2 2 3 2
Interest and other income [1] 10 6 20 12
Total revenue [1] 13 8 24 15
Broking [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 996 1,057 1,932 2,134
Broking [Member] | HWC [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 150 279 296 614
Broking [Member] | R&B [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 846 778 1,636 1,520
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 804 748 1,602 1,515
Consulting [Member] | HWC [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 704 651 1,383 1,313
Consulting [Member] | R&B [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 99 97 218 201
Consulting [Member] | Corporate Segment [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax [1] 1 0 1 1
Outsourced administration [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 257 274 546 570
Outsourced administration [Member] | HWC [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 240 257 508 523
Outsourced administration [Member] | R&B [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 17 17 38 47
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 135 120 271 255
Other [Member] | HWC [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 78 64 141 128
Other [Member] | R&B [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 57 56 130 127
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 2,192 2,199 4,351 4,474
Service [Member] | HWC [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 1,172 1,251 2,328 2,578
Service [Member] | R&B [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 1,019 948 2,022 1,895
Service [Member] | Corporate Segment [Member]        
Disaggregation Of Revenue [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax [1] $ 1 $ 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.