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Revenue - Narrative (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2022 USD ($) installment | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($)
Disaggregation of Revenue [Line Items] |  |  |
Percentage of total revenue | 2.00% |  |
Number of installments for brokerage revenue | installment | 4 |  |
Contract with customer, performance obligation satisfied in previous period | $ 83 | $ 84 | $ 97
Capitalized contract cost, amortization | 1,600 | 1,500 | $ 1,300
Deferred Implementation Costs |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Capitalized contract cost, net | 19 | 24 |
Costs to Obtain |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Capitalized contract cost, net | 328 | 290 |
Costs to Fulfill |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Capitalized contract cost, net | $ 320 | $ 316 |
Mercer |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Consulting revenue, payment terms (in days) | 30 days |  |
Revenue, remaining performance obligation | $ 231 |  |
Minimum | Oliver Wyman Group |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Consulting revenue, payment terms (in days) | 30 days |  |
Maximum | Oliver Wyman Group |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Consulting revenue, payment terms (in days) | 60 days |  |
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 |  |
Health Brokerage and Consulting Services |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue from contract with customer, percent | 59.00% |  |

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