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Revenue - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2018 | Mar. 31, 2018 | Jan. 01, 2018 | Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |
Retained earnings | $ 14,347 |  | $ 13,504 | $ 13,140
Percentage of total revenue | 1.00% |  |  |
Cumulative catch-up adjustment to revenue, change in measure of progress | $ 331 |  |  |
Contract with customer, asset, reclassified to receivable | 347 |  |  |
Contract with customer, liability, revenue recognized | 582 |  |  |
Contract with customer, performance obligation satisfied in previous period | 51 |  |  |
Capitalized contract cost, amortization | $ 998 |  |  |
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 | $ 32 |  |  |
Mercer Consulting Group |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |
Revenue, remaining performance obligation | 486 |  |  |
Oliver Wyman Group Consulting Group |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |
Revenue, remaining performance obligation | 2 |  |  |
Deferred Implementation Costs |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |
Capitalized contract cost, net | 41 |  |  |
Costs to Obtain |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |
Capitalized contract cost, net | 206 |  |  |
Costs to Fulfill |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |
Capitalized contract cost, net | $ 227 |  |  |

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- Definition Amount of increase (decrease) in revenue recognized for cumulative catch-up adjustment from change in measure of progress which increases (decreases) right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130551-203045 + Details
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