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Revenue - Narrative (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2020 USD ($) installment | Dec. 31, 2019 USD ($)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Percentage of total revenue | 1.00% |
Number of installments for brokerage revenue | installment | 4 |
Cumulative catch-up adjustment to revenue, change in measure of progress | $ 311 | $ 437
Contract with customer, asset, reclassified to receivable | 284 | 342
Cash received for performance obligations not yet fulfilled | 615 |
Contract with customer, liability, revenue recognized | 527 | 531
Revenue reduction for estimated commission revenue | 42 |
Contract with customer, performance obligation satisfied in previous period | 97 | 79
Capitalized contract cost, amortization | 1,300 | 1,200
Deferred implementation costs |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Capitalized contract cost, net | 29 | 30
Costs to obtain |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Capitalized contract cost, net | 253 | 222
Costs to fulfill |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Capitalized contract cost, net | $ 296 | $ 262
Mercer |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Consulting revenue, payment terms (in days) | 30 days |
Revenue, remaining performance obligation | $ 184 |
Oliver Wyman Group |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Revenue, remaining performance obligation | 6 |
Marsh |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Revenue, remaining performance obligation | $ 38 |
Minimum | Oliver Wyman Group |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Consulting revenue, payment terms (in days) | 30 days |
Maximum | Oliver Wyman Group |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Consulting revenue, payment terms (in days) | 60 days |
Risk and insurance services |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Insurance brokerage commissions, payment terms (in days) | 30 days |
Risk and insurance services | Minimum |  |
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 |
Risk and insurance services | Maximum |  |
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 |
Health brokerage and consulting services |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |
Revenue from contract with customer, percent | 60.00% |

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