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Revenue - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2018 | Jan. 01, 2018 | Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |
Retained earnings | $ 13,812 | $ 13,504 | $ 13,140
Contract with customer, asset, additions | 128 |  |
Contract with customer, asset, reclassified to receivable | 87 |  |
Contract with customer, liability, revenue recognized | 181 |  |
Performance obligation satisfied in previous period | 14 |  |
Capitalized contract cost, amortization | 296 |  |
Accounting Standards Update 2014-09 |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |
Retained earnings | $ 364 | $ 364 |
Health Brokerage and Consulting |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |
Revenue from contract with customer, percent | 70.00% |  |
Marsh Insurance Group |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |
Revenue, remaining performance obligation | $ 24 |  |
Mercer Consulting Group |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |
Revenue, remaining performance obligation | 575 |  |
Oliver Wyman Group Consulting Group |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |
Revenue, remaining performance obligation | 8 |  |
Deferred Implementation Costs |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |
Capitalized contract cost, net | 39 |  |
Costs to Obtain |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |
Capitalized contract cost, net | 194 |  |
Costs to Fulfill |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |
Capitalized contract cost, net | $ 171 |  |

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