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Acquisitions and Dispositions (Narrative) (Details) £ / shares in Units, colleague in Thousands, $ in Millions, £ in Billions |  |  |  | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | ---
Jun. 01, 2019 USD ($) | Apr. 01, 2019 USD ($) | Apr. 01, 2019 GBP (£) colleague £ / shares | Sep. 20, 2018 GBP (£) | Jun. 30, 2019 USD ($) | Jun. 30, 2018 USD ($) | Jun. 30, 2019 USD ($) acquisition | Jun. 30, 2018 USD ($) acquisition | Jan. 01, 2019 | Dec. 31, 2018
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Assumption of debt |  |  |  |  |  |  | $ 2,718 | $ 28 |  |
Total consideration |  |  |  |  |  |  | 5,925 | 192 |  |
Cash |  |  |  |  |  |  | 5,859 | 160 |  |
Estimated fair value of deferred/contingent consideration |  |  |  |  |  |  | 66 | 32 |  |
Deferred purchase consideration from prior years' acquisitions |  |  |  |  |  |  |  | 53 |  |
Contingent consideration from prior year's acquisitions |  |  |  |  |  |  | 16 | 32 |  |
Increase (decrease) in costs |  |  |  |  | $ (151) |  | (169) | 658 |  |
Jardine Lloyd Thompson Group plc |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Business acquisition, share price (in usd per share) | £ / shares |  |  | £ 19.15 |  |  |  |  |  |  |
Equity interests issued and issuable |  | $ 5,600 | £ 4.3 |  |  |  |  |  |  |
Assumption of debt |  | $ 1,000 |  |  |  |  |  |  |  |
Entity number of employees | colleague |  |  | 10 |  |  |  |  |  |  |
Total consideration |  |  |  | £ 5.2 |  |  | 5,568 |  |  |
Cash |  |  |  |  |  |  | 5,568 |  |  |
Estimated fair value of deferred/contingent consideration |  |  |  |  |  |  | 0 |  |  |
Business combination, acquisition related costs |  |  |  |  | 84 |  | 95 |  |  |
Current Fiscal Period Acquisitions |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Total consideration |  |  |  |  |  |  | 5,925 |  |  |
Cash |  |  |  |  |  |  | 5,859 |  |  |
Estimated fair value of deferred/contingent consideration |  |  |  |  |  |  | 66 |  |  |
Revenue related to acquisitions |  |  |  |  | 486 | $ 11 | 496 | 14 |  |
Operating (loss) income related to acquisitions |  |  |  |  | $ 16 | $ (1) | 18 | (2) |  |
Prior Fiscal Periods Acquisitions |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Deferred purchase consideration from prior years' acquisitions |  |  |  |  |  |  | 23 | 53 |  |
Contingent consideration from prior year's acquisitions |  |  |  |  |  |  | $ 45 | $ 41 |  |
Risk and Insurance Services Segment |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  |  |  |  |  |  | 4 | 12 |  |
Consulting Segment |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  |  |  |  |  |  |  | 8 |  |
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 India |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Percentage of ownership in equity investment |  |  |  |  |  |  |  |  | 49.00% | 26.00%
JLT’s Global Aerospace Business | Disposal Group, Disposed of by Sale, Not Discontinued Operations |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Disposal group, consideration | $ 165 |  |  |  |  |  |  |  |  |
Contingent consideration receivable | $ 65 |  |  |  |  |  |  |  |  |

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