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Fair Value Measurements (Narrative) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Carrying value of investment | $ 328 |  | $ 328 |  | $ 280
Equity securities | 74 |  | 74 |  | 72
Equity investments without readily determinable market value | 33 |  | 33 |  | 33
Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Proceeds from contingent purchase consideration, asset |  |  | $ 90 |  |
Minimum |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Revenue target period (in years) |  |  | 2 years |  |
Maximum |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Revenue target period (in years) |  |  | 4 years |  |
Private Insurance and Consulting |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Carrying value of investment | 178 |  | $ 178 |  | 169
Private Equity Funds |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Carrying value of investment | 150 |  | 150 |  | 111
Equity method income (loss) | 13 | $ 2 | 40 | $ (6) |
Alexander Forbes Group Holdings Limited |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Investments, fair value | $ 55 |  | $ 55 |  | $ 54
Number of shares sold (shares) |  |  |  |  | 242
Money Market Funds |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Share price (per share amount) | $ 1.00 |  | $ 1.00 |  |

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