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Acquisitions (Narrative) (Details) $ in Millions |  | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jun. 25, 2018 USD ($) colleague | Jun. 30, 2018 USD ($) | Jun. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) acquisition | Jun. 30, 2017 USD ($) | Dec. 31, 2017 acquisition
Business Acquisition [Line Items] |  |  |  |  |  |
Total consideration |  |  |  | $ 192 | $ 510 |
Cash |  |  |  | 160 | 420 |
Estimated fair value of deferred/contingent consideration |  |  |  | 32 | 90 |
Contingent consideration from prior year's acquisitions |  |  |  | 32 | 60 |
Current Fiscal Period Acquisitions |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Total consideration |  |  |  | 192 |  |
Cash |  |  |  | 160 |  |
Estimated fair value of deferred/contingent consideration |  |  |  | 32 |  |
Revenue related to acquisitions |  | $ 11 | $ 34 | 14 | 63 |
Operating (loss) income related to acquisitions |  | $ (1) | $ 5 | (2) | 15 |
Prior Fiscal Periods Acquisitions |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Deferred purchase consideration from prior years' acquisitions |  |  |  | 53 | 37 |
Contingent consideration from prior year's acquisitions |  |  |  | $ 41 | $ 65 |
Wortham Insurance |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Revenue | $ 130 |  |  |  |  |
Number of colleagues | colleague | 530 |  |  |  |  |
Risk and Insurance Services Segment |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  |  |  | 7 |  | 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 |  |

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