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Revenue - Narrative (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2021 USD ($) installment | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |
Percentage of total revenue | 2.00% |  |
Number of installments for brokerage revenue | installment | 4 |  |
Cumulative catch-up adjustment to revenue, change in measure of progress | $ 547 | $ 311 |
Contract with customer, asset, reclassified to receivable | 493 | 284 |
Cash received for performance obligations not yet fulfilled | 642 |  |
Contract with customer, liability, revenue recognized | 539 | 527 |
Contract with customer, performance obligation satisfied in previous period | 84 | 97 | $ 79
Capitalized contract cost, amortization | 1,500 | 1,300 | $ 1,200
Deferred implementation costs |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |
Capitalized contract cost, net | 24 | 29 |
Costs to obtain |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |
Capitalized contract cost, net | 290 | 253 |
Costs to fulfill |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |
Capitalized contract cost, net | $ 316 | $ 296 |
Mercer |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |
Consulting revenue, payment terms (in days) | 30 days |  |
Revenue, remaining performance obligation | $ 187 |  |
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 | Operating Segments |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |
Gain (loss) on consolidation of business | $ 267 |  |
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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