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Acquisitions and Dispositions (Narrative) (Details) £ / shares in Units, $ in Millions, £ in Billions |  |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Apr. 01, 2019 USD ($) | Apr. 01, 2019 GBP (£) £ / shares | Sep. 20, 2018 GBP (£) | Mar. 31, 2019 USD ($) acquisition | Mar. 31, 2018 USD ($) | Dec. 31, 2018 acquisition | Jan. 01, 2019
Business Acquisition [Line Items] |  |  |  |  |  |  |
Total consideration |  |  |  | $ 177 | $ 36 |  |
Cash |  |  |  | 142 | 29 |  |
Estimated fair value of deferred/contingent consideration |  |  |  | 35 | 7 |  |
Deferred purchase consideration from prior years' acquisitions |  |  |  | 23 | 40 |  |
Contingent consideration from prior year's acquisitions |  |  |  | 6 | 30 |  |
Assumption of debt |  |  |  | 5 | 4 |  |
Current Fiscal Period Acquisitions |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Total consideration |  |  |  | 177 |  |  |
Cash |  |  |  | 142 |  |  |
Estimated fair value of deferred/contingent consideration |  |  |  | 35 |  |  |
Revenue related to acquisitions |  |  |  | 4 | 3 |  |
Operating (loss) income related to acquisitions |  |  |  | 1 | (1) |  |
Prior Fiscal Periods Acquisitions |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Deferred purchase consideration from prior years' acquisitions |  |  |  |  | 40 |  |
Contingent consideration from prior year's acquisitions |  |  |  | $ 35 | $ 40 |  |
Jardine Lloyd Thompson Group plc |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Total consideration | £ |  |  | £ 5.2 |  |  |  |
Risk and Insurance Services Segment |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  |  |  | 2 |  | 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 |  |  |
Subsequent Event | Jardine Lloyd Thompson Group plc |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Total consideration | £ |  | £ 5.2 |  |  |  |  |
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 |  |  |  |  |  |
Marsh India |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Percentage of ownership in equity investment |  |  |  |  |  | 26.00% | 49.00%

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