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Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | $ 15,698 | $ 13,376 | $ 12,479
U.S. |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 7,666 | 5,923 | 5,666
Americas other than U.S. |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 1,335 | 1,260 | 1,137
U.K. |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 1,975 | 1,819 | 1,660
Ireland |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 146 | 113 | 99
Europe, Middle East, & Africa other than U.K. and Ireland |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 2,900 | 2,672 | 2,443
Asia Pacific |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 1,676 | 1,589 | 1,474
Operating Segments | Risk Capital |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 10,517 | 9,524 | 8,905
Operating Segments | Risk Capital | U.S. |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 4,697 | 4,067 | 3,910
Operating Segments | Risk Capital | Americas other than U.S. |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 954 | 909 | 837
Operating Segments | Risk Capital | U.K. |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 1,266 | 1,186 | 1,068
Operating Segments | Risk Capital | Ireland |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 78 | 61 | 53
Operating Segments | Risk Capital | Europe, Middle East, & Africa other than U.K. and Ireland |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 2,176 | 2,036 | 1,877
Operating Segments | Risk Capital | Asia Pacific |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 1,346 | 1,265 | 1,160
Operating Segments | Human Capital |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 5,209 | 3,864 | 3,591
Operating Segments | Human Capital | U.S. |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 2,997 | 1,868 | 1,773
Operating Segments | Human Capital | Americas other than U.S. |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 381 | 351 | 300
Operating Segments | Human Capital | U.K. |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 709 | 633 | 592
Operating Segments | Human Capital | Ireland |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 68 | 52 | 46
Operating Segments | Human Capital | Europe, Middle East, & Africa other than U.K. and Ireland |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 724 | 636 | 566
Operating Segments | Human Capital | Asia Pacific |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 330 | 324 | 314
Operating Segments | Commercial Risk Solutions | Risk Capital |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 7,861 | 7,043 | 6,715
Operating Segments | Reinsurance Solutions | Risk Capital |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 2,656 | 2,481 | 2,190
Operating Segments | Health Solutions | Human Capital |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 3,335 | 2,433 | 2,224
Operating Segments | Wealth Solutions | Human Capital |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 1,874 | 1,431 | 1,367
Eliminations |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | (28) | (12) | (17)
Eliminations | Risk Capital |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 16 | 6 | 9
Eliminations | Human Capital |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 12 | 6 | 8
Corporate/Eliminations |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | (28) | (12) | (17)
Corporate/Eliminations | U.S. |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | (28) | (12) | (17)
Corporate/Eliminations | Americas other than U.S. |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 0 | 0 | 0
Corporate/Eliminations | U.K. |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 0 | 0 | 0
Corporate/Eliminations | Ireland |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 0 | 0 | 0
Corporate/Eliminations | Europe, Middle East, & Africa other than U.K. and Ireland |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 0 | 0 | 0
Corporate/Eliminations | Asia Pacific |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | $ 0 | $ 0 | $ 0

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