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Revenue - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2018 | Jan. 01, 2018 | Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |
Retained earnings | $ 14,131 | $ 14,131 | $ 13,504 | $ 13,140
Contract with customer, asset, additions |  | 215 |  |
Contract with customer, asset, reclassified to receivable |  | 166 |  |
Contract with customer, liability, revenue recognized |  | 334 |  |
Performance obligation satisfied in previous period |  | 36 |  |
Capitalized contract cost, amortization | 261 | 557 |  |
Accounting Standards Update 2014-09 |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |
Retained earnings | 364 | $ 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 | 48 | $ 48 |  |
Mercer Consulting Group |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |
Revenue, remaining performance obligation | 450 | 450 |  |
Oliver Wyman Group Consulting Group |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |
Revenue, remaining performance obligation | 2 | 2 |  |
Deferred Implementation Costs |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |
Capitalized contract cost, net | 39 | 39 |  |
Costs to Obtain |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |
Capitalized contract cost, net | 196 | 196 |  |
Costs to Fulfill |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |
Capitalized contract cost, net | $ 154 | $ 154 |  |

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