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Goodwill and Other Intangibles (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020 | Dec. 31, 2019
Segment Reporting Information [Line Items] |  |  |  |
Goodwill | $ 15,458 | $ 14,412 | $ 15,517 | $ 14,671
Aggregate amortization expense | 100 | $ 86 |  |
Risk and Insurance Services Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Goodwill | 11,700 |  |  |
Consulting Segment |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Goodwill | $ 3,800 |  |  |

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