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Revenue from Contracts with Customers - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2025 | Mar. 31, 2024
Disaggregation of Revenue [Line Items] |  |
Total revenue | $ 4,729 | $ 4,070
U.S. |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 2,002 | 1,512
Americas other than U.S. |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 374 | 323
U.K. |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 621 | 583
Ireland |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 45 | 33
Europe, Middle East, & Africa other than U.K. and Ireland |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 1,262 | 1,209
Asia Pacific |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 425 | 410
Operating Segments | Risk Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 3,191 | 2,975
Operating Segments | Human Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 1,545 | 1,103
Operating Segments | U.S. | Risk Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 1,221 | 1,078
Operating Segments | U.S. | Human Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 788 | 442
Operating Segments | Americas other than U.S. | Risk Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 254 | 227
Operating Segments | Americas other than U.S. | Human Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 120 | 96
Operating Segments | U.K. | Risk Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 430 | 411
Operating Segments | U.K. | Human Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 191 | 172
Operating Segments | Ireland | Risk Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 22 | 16
Operating Segments | Ireland | Human Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 23 | 17
Operating Segments | Europe, Middle East, & Africa other than U.K. and Ireland | Risk Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 946 | 928
Operating Segments | Europe, Middle East, & Africa other than U.K. and Ireland | Human Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 316 | 281
Operating Segments | Asia Pacific | Risk Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 318 | 315
Operating Segments | Asia Pacific | Human Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 107 | 95
Eliminations |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | (7) | (8)
Eliminations | Risk Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 5 | 6
Eliminations | Human Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 2 | 2
Corporate/Eliminations |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | (7) | (8)
Corporate/Eliminations | U.S. |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | (7) | (8)
Corporate/Eliminations | Americas other than U.S. |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 0 | 0
Corporate/Eliminations | U.K. |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 0 | 0
Corporate/Eliminations | Ireland |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 0 | 0
Corporate/Eliminations | Europe, Middle East, & Africa other than U.K. and Ireland |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 0 | 0
Corporate/Eliminations | Asia Pacific |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 0 | 0
Commercial Risk Solutions | Operating Segments | Risk Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 2,002 | 1,808
Reinsurance Solutions | Operating Segments | Risk Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 1,189 | 1,167
Health Solutions | Operating Segments | Human Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 1,026 | 733
Wealth Solutions | Operating Segments | Human Capital |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | $ 519 | $ 370

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