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Revenue - Narrative (Details) - USD ($) $ in Millions | 9 Months Ended
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Sep. 30, 2020 | Sep. 30, 2019
Disaggregation of Revenue [Line Items] |  |
Revenue recognized that was included in the contract liability balance at the beginning of the period | $ 420 | $ 378
Performance obligation satisfied in previous period | 84 | $ 67
Marsh Insurance Group |  |
Disaggregation of Revenue [Line Items] |  |
Revenue, remaining performance obligation | 46 |
Mercer Consulting Group |  |
Disaggregation of Revenue [Line Items] |  |
Revenue, remaining performance obligation | 181 |
Oliver Wyman Group Consulting Group |  |
Disaggregation of Revenue [Line Items] |  |
Revenue, remaining performance obligation | $ 2 |

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- Definition Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130549-203045 + 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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- Definition Amount of transaction price allocated to performance obligation that has not been recognized as revenue. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 13 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130556-203045 + Details
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- Details
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Period Type: |

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- Details
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Period Type: |

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- Details
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