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Acquisitions and Dispositions (Narrative) (Details) £ / shares in Units, colleague in Thousands, $ in Millions, £ in Billions |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | ---
Sep. 18, 2018 USD ($) colleague | Sep. 18, 2018 GBP (£) colleague £ / shares | Jul. 31, 2018 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) acquisition | Sep. 30, 2017 USD ($) | Dec. 31, 2017 acquisition
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Total consideration |  |  |  |  |  | $ 661 | $ 734 |
Cash |  |  |  |  |  | 553 | 640 |
Estimated fair value of deferred/contingent consideration |  |  |  |  |  | 108 | 94 |
Contingent consideration from prior year's acquisitions |  |  |  |  |  | 47 | 80 |
Current Fiscal Period Acquisitions |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Total consideration |  |  |  |  |  | 661 |  |
Cash |  |  |  |  |  | 553 |  |
Estimated fair value of deferred/contingent consideration |  |  |  |  |  | 108 |  |
Revenue related to acquisitions |  |  |  | $ 36 | $ 50 | 40 | 103 |
Operating (loss) income related to acquisitions |  |  |  | $ (5) | $ (7) | (1) | 16 |
Jardine Lloyd Thompson Group plc |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Total consideration | $ 5,200 |  |  |  |  |  |  |
Revenue | 2,000 |  |  |  |  |  |  |
Equity interests issued and issuable | $ 5,600 | £ 4.3 |  |  |  |  |  |
Business combination, exchange rate | 1.31 | 1.31 |  |  |  |  |  |
Threshold percentage value of majority shareholders | 75.00% | 75.00% |  |  |  |  |  |
Number of colleagues | colleague | 10 | 10 |  |  |  |  |  |
Business acquisition, share price | £ / shares |  | £ 19.15 |  |  |  |  |  |
Prior Fiscal Periods Acquisitions |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Deferred purchase consideration from prior years' acquisitions |  |  |  |  |  | 59 | 47 |
Contingent consideration from prior year's acquisitions |  |  |  |  |  | $ 76 | $ 107 |
Risk and Insurance Services Segment |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  |  |  |  |  | 8 |  | 7
Consulting Segment |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  |  |  |  |  | 5 |  | 3
Minimum |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Revenue target period (in years) |  |  |  |  |  | 2 years |  |
Minimum | Current Fiscal Period Acquisitions |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Revenue target period (in years) |  |  |  |  |  | 2 years |  |
Maximum |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Revenue target period (in years) |  |  |  |  |  | 4 years |  |
Maximum | Current Fiscal Period Acquisitions |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Revenue target period (in years) |  |  |  |  |  | 4 years |  |
Marsh ClearSight |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Gain on disposition of business |  |  | $ 46 |  |  |  |  |

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