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Acquisitions (Narrative) (Details) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2017 USD ($) | Sep. 30, 2017 USD ($) acquisition | Sep. 30, 2016 USD ($) | Dec. 31, 2016 acquisition
Business Acquisition [Line Items] |  |  |  |
Total consideration |  | $ 734 | $ 119 |
Cash |  | 640 | 90 |
Estimated fair value of deferred/contingent consideration |  | 94 | 29 |
Contingent consideration from prior year's acquisitions |  | 80 | 43 |
Current Fiscal Period Acquisitions |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Total consideration |  | 734 |  |
Cash |  | 640 |  |
Estimated fair value of deferred/contingent consideration |  | 94 |  |
Revenue related to acquisitions | $ 40 | 103 |  |
Operating income related to acquisitions | $ (1) | 16 |  |
Prior Fiscal Periods Acquisitions |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Deferred purchase consideration from prior years' acquisitions |  | 47 | 53 |
Contingent consideration from prior year's acquisitions |  | $ 107 | $ 85 |
Risk and Insurance Services Segment |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  | 7 |  | 9
Consulting Segment |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  | 1 |  | 6
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 |  |

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