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Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Oct. 31, 2023 | Mar. 31, 2022 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Disaggregation of Revenue [Line Items] |  |  |  |  |
Revenue |  |  | $ 22,736 | $ 20,720 | $ 19,820
Loss on consolidation of entity |  | $ 39 | 0 | 39 | 0
Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Revenue |  |  | 22,798 | 20,784 | 19,874
Mercer U.S. Affinity Business | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Pre-tax gain (loss) on sale |  |  |  |  | 50
Risk and Insurance Services |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Fiduciary interest income |  |  | 453 | 120 | 15
Risk and Insurance Services | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Fiduciary interest income |  |  | 453 | 120 | 15
Revenue |  |  | 14,089 | 12,645 | 12,085
Pre-tax gain (loss) on sale |  |  |  |  | 50
Risk and Insurance Services | Subtotal | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Revenue, excluding fiduciary interest income |  |  | 13,636 | 12,525 | 12,070
Risk and Insurance Services | Marsh | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Revenue, excluding fiduciary interest income |  |  | 11,378 | 10,505 | 10,203
Revenue |  |  | 11,657 | 10,585 | 10,214
Risk and Insurance Services | Guy Carpenter |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Gain from the settlement | $ 58 |  | 58 |  |
Risk and Insurance Services | Guy Carpenter | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Revenue, excluding fiduciary interest income |  |  | 2,258 | 2,020 | 1,867
Revenue |  |  | 2,432 | 2,060 | 1,871
Risk and Insurance Services | Marsh India | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Gain (loss) on consolidation of business |  |  |  |  | 267
Risk and Insurance Services | Total International | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Revenue, excluding fiduciary interest income |  |  | 5,116 | 4,714 | 4,861
Risk and Insurance Services | EMEA | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Revenue, excluding fiduciary interest income |  |  | 3,262 | 2,997 | 3,236
Risk and Insurance Services | Asia Pacific | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Revenue, excluding fiduciary interest income |  |  | 1,295 | 1,215 | 1,172
Risk and Insurance Services | Latin America | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Revenue, excluding fiduciary interest income |  |  | 559 | 502 | 453
Risk and Insurance Services | U.S./Canada | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Revenue, excluding fiduciary interest income |  |  | 6,262 | 5,791 | 5,342
Risk and Insurance Services | RUSSIAN FEDERATION | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Loss on consolidation of entity |  |  |  | 27 |
Consulting | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Revenue |  |  | 8,709 | 8,139 | 7,789
Consulting | Wealth | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Revenue |  |  | 2,507 | 2,366 | 2,509
Consulting | Health | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Revenue |  |  | 2,061 | 2,017 | 1,855
Consulting | Career | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Revenue |  |  | 1,019 | 962 | 890
Consulting | Mercer | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Revenue |  |  | 5,587 | 5,345 | 5,254
Consulting | Oliver Wyman Group | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Revenue |  |  | 3,122 | 2,794 | $ 2,535
Consulting | Mercer U.S. Affinity Business | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Pre-tax gain (loss) on sale |  |  |  | 112 |
Consulting | Small Individual Financial Advisory Business | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Pre-tax gain (loss) on sale |  |  | $ (17) |  |
Consulting | RUSSIAN FEDERATION | Operating Segments |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Loss on consolidation of entity |  |  |  | $ 12 |

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