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Revenue - Narrative (Details) - USD ($) $ in Millions |  | 6 Months Ended
--- | --- | ---
Apr. 01, 2019 | Jun. 30, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Percentage of total revenue |  | 1.00%
Increases as a result of cumulative catch-up adjustments arising from changes in the estimate of the stage of completion, excluding amounts transferred to receivables during the period |  | $ 282
Transferred to receivables from contract assets recognized at the beginning of the period |  | 176
Cash received for performance obligations not yet fulfilled |  | 362
Revenue recognized that was included in the contract liability balance at the beginning of the period |  | 307
Performance obligation satisfied in previous period |  | 36
Marsh Insurance Group |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Revenue, remaining performance obligation |  | $ 22
Mercer Consulting Group |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Consulting revenue, payment terms (in days) |  | 30 days
Revenue, remaining performance obligation |  | $ 323
Oliver Wyman Group Consulting Group |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Revenue, remaining performance obligation |  | $ 3
Risk and Insurance Services Segment |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Insurance brokerage commissions, payment terms (in days) |  | 30 days
Health Brokerage and Consulting |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Revenue from contract with customer, percent |  | 62.00%
Minimum | Oliver Wyman Group Consulting Group |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Consulting revenue, payment terms (in days) |  | 30 days
Minimum | Risk and Insurance Services Segment |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Estimated brokerage revenue, payment terms (in months) |  | 12 months
Reinsurance brokerage revenue, payment terms (in months) |  | 12 months
Maximum | Oliver Wyman Group Consulting Group |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Consulting revenue, payment terms (in days) |  | 60 days
Maximum | Risk and Insurance Services Segment |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Estimated brokerage revenue, payment terms (in months) |  | 18 months
Reinsurance brokerage revenue, payment terms (in months) |  | 18 months
Jardine Lloyd Thompson Group plc |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Increase for contract acquired in business combination | $ 62 |
Increase for contract acquired in business combination | $ 47 |

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