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Revenue - Narrative (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2023 USD ($) installment | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($)
Disaggregation of Revenue [Line Items] |  |  |
Percentage of total revenue | 1.00% |  |
Number of installments for brokerage revenue | installment | 4 |  |
Contract with customer, performance obligation satisfied in previous period | $ 71 | $ 83 | $ 84
Capitalized contract cost, amortization | 1,800 | 1,600 | $ 1,500
Deferred Implementation Costs |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Capitalized contract cost, net | 10 | 19 |
Costs to Obtain |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Capitalized contract cost, net | 362 | 328 |
Costs to Fulfill |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Capitalized contract cost, net | $ 370 | $ 320 |
Mercer |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Consulting revenue, payment terms (in days) | 30 days |  |
Minimum |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Capitalized contract cost, amortization period | 3 months |  |
Maximum |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Capitalized contract cost, amortization period | 6 months |  |
Risk and Insurance Services |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Insurance brokerage commissions, payment terms (in days) | 30 days |  |
Risk and Insurance Services | Minimum |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Estimated brokerage revenue, payment terms (in months) | 12 months |  |
Reinsurance brokerage revenue, payment terms (in months) | 12 months |  |
Risk and Insurance Services | Maximum |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Estimated brokerage revenue, payment terms (in months) | 18 months |  |
Reinsurance brokerage revenue, payment terms (in months) | 18 months |  |
Consulting Segment | Mercer |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue from contract with customer, percent | 57.00% |  |
Consulting Segment | Minimum | Oliver Wyman Group |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Consulting revenue, payment terms (in days) | 30 days |  |
Consulting Segment | Maximum | Oliver Wyman Group |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Consulting revenue, payment terms (in days) | 60 days |  |

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