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Goodwill and Other Intangibles (Narrative) (Details) - USD ($) $ in Millions | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2016 | Dec. 31, 2015
Segment Reporting Information [Line Items] |  |  |  |
Goodwill | $ 9,100 | $ 7,975 | $ 8,369 | $ 7,889
Aggregate amortization expense | 122 | $ 99 |  |
Risk and Insurance Services Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Goodwill | 6,500 |  |  |
Consulting Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Goodwill | $ 2,600 |  |  |

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