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Revenue - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2020 | Jun. 30, 2020 | Jun. 30, 2019
Disaggregation of Revenue [Line Items] |  |  |
Revenue recognized that was included in the contract liability balance at the beginning of the period |  | $ 418 | $ 307
Performance obligation satisfied in previous period |  | 67 | $ 36
Revenue reduction for estimated commission revenue | $ 36 |  |
Marsh Insurance Group |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue, remaining performance obligation | 51 | 51 |
Mercer Consulting Group |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue, remaining performance obligation | 220 | 220 |
Oliver Wyman Group Consulting Group |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue, remaining performance obligation | 2 | $ 2 |
Risk and Insurance Services Segment | Marsh Insurance Group |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue reduction for estimated commission revenue | 33 |  |
Consulting Segment | Mercer Consulting Group | Health |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenue reduction for estimated commission revenue | $ 3 |  |

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- Definition Amount of increase (decrease) in revenue recognized for cumulative catch-up adjustment from change in estimate of transaction price which (increases) decreases obligation to transfer good or service to customer for which consideration from customer has been received or is due. Includes, but is not limited to, change in assessment of whether estimate of variable consideration is constrained. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130551-203045 + Details
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- Definition Amount of revenue recognized from performance obligation satisfied or partially satisfied in previous reporting periods. Includes, but is not limited to, change in transaction price. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 12A -URI http://asc.fasb.org/extlink&oid=121604090&loc=SL108322424-203045 + Details
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