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Acquisitions and Dispositions (Narrative) (Details) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2021 USD ($) | Sep. 30, 2020 USD ($) | Sep. 30, 2021 USD ($) acquisition | Sep. 30, 2020 USD ($) | Dec. 31, 2020 acquisition
Business Acquisition [Line Items] |  |  |  |  |
Total consideration |  |  | $ 546 | $ 742 |
Cash paid |  |  | 405 | 579 |
Estimated fair value of deferred/contingent consideration |  |  | 141 | 163 |
Deferred purchase consideration from prior years' acquisitions |  |  | 84 | 60 |
Contingent consideration from prior year's acquisitions |  |  | 26 | 65 |
Current Fiscal Period Acquisitions |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Total consideration |  |  | 546 |  |
Cash paid |  |  | 405 |  |
Estimated fair value of deferred/contingent consideration |  |  | 141 |  |
Revenue related to acquisitions | $ 38 |  | 73 |  |
Operating income (loss) related to acquisitions | $ 3 |  | 4 |  |
Prior Fiscal Periods Acquisitions |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Estimated fair value of deferred/contingent consideration |  |  | $ 72 |  |
Deferred purchase consideration from prior years' acquisitions |  |  |  | 60 |
Revenue related to acquisitions |  | $ 52 |  | 115 |
Operating income (loss) related to acquisitions |  | $ (8) |  | 3 |
Contingent consideration from prior year's acquisitions |  |  |  | 101 |
Risk and Insurance Services Segment |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  |  | 3 |  | 7
Minimum |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Revenue target period (in years) |  |  | 2 years |  |
Maximum |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Revenue target period (in years) |  |  | 4 years |  |
Certain Businesses Primarily in the U.S., U.K. and Canada |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Disposal group, consideration |  |  | $ 84 | $ 93 |
Gain on disposal |  |  | $ 50 |  |

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