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Acquisitions/Dispositions (Narrative) (Details) £ / shares in Units, colleague in Thousands, $ in Millions, £ in Billions |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 18, 2018 USD ($) colleague | Sep. 18, 2018 GBP (£) colleague £ / shares | Sep. 30, 2018 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2018 USD ($) acquisition | Dec. 31, 2017 USD ($) acquisition | Dec. 31, 2016 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |  |
Total consideration |  |  |  |  | $ 1,040 | $ 777 |
Cash |  |  |  |  |  | 668 |
Estimated fair value of deferred/contingent consideration |  |  |  |  | 133 | 109 | $ 36
Deferred purchase consideration from prior years' acquisitions |  |  |  |  | 62 | 55 | 54
Contingent payments for acquisitions |  |  |  |  | 55 | 81 | 44
Deferred purchase consideration |  |  |  |  | 117 | 136 | 98
Revenue related to acquisitions |  |  |  |  |  | 156 | 25
Net operating income related to acquisitions |  |  |  |  |  | 19 | $ 4
Acquisition related costs |  |  |  |  | 7 | 3 |
Gain on disposition of business |  |  | $ 46 | $ 6 |  |  |
Current Fiscal Period Acquisitions |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Total consideration |  |  |  |  | 1,043 |  |
Cash |  |  |  |  | 910 |  |
Estimated fair value of deferred/contingent consideration |  |  |  |  | 133 |  |
Revenue related to acquisitions |  |  |  |  | 120 |  |
Net operating income related to acquisitions |  |  |  |  | 2 |  |
Prior Fiscal Periods Acquisitions |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Estimated fair value of deferred/contingent consideration |  |  |  |  | 91 |  |
Deferred purchase consideration from prior years' acquisitions |  |  |  |  |  | 55 |
Contingent payments for acquisitions |  |  |  |  |  | $ 108 |
Deferred purchase consideration |  |  |  |  | $ 62 |  |
Jardine Lloyd Thompson Group plc |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Total consideration | £ |  | £ 5.2 |  |  |  |  |
Revenues | $ 2,000 |  |  |  |  |  |
Number of colleagues | colleague | 10 | 10 |  |  |  |  |
Share price | £ / shares |  | £ 19.15 |  |  |  |  |
Consideration transferred, equity interests issued and issuable | $ 5,600 | £ 4.3 |  |  |  |  |
Exchange rate | 1.31 | 1.31 |  |  |  |  |
Risk and Insurance Services Segment |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Number of acquisitions made | acquisition |  |  |  |  | 12 | 7 |
Consulting Segment |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Number of acquisitions made | acquisition |  |  |  |  | 8 | 3 |
Minimum |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Revenue target period (in years) |  |  |  |  | 2 years |  |
Maximum |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Revenue target period (in years) |  |  |  |  | 4 years |  |

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