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Fair Value Measurements (Narrative) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
May 31, 2020 | Feb. 29, 2020 | Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Carrying value of investment |  |  | $ 289 |  | $ 280
Equity securities |  |  | 74 |  | 72
Equity investments without readily determinable market value |  |  | $ 33 |  | $ 33
Minimum |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Revenue target period (in years) |  |  | 2 years |  | 2 years
Maximum |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Revenue target period (in years) |  |  | 4 years |  | 4 years
Private Insurance and Consulting |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Carrying value of investment |  |  | $ 172 |  | $ 169
Private Equity Funds |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Carrying value of investment |  |  | 117 |  | 111
Equity method income (loss) |  |  | 10 | $ (1) |
Alexander Forbes Group Holdings Limited |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Investments, fair value |  |  | $ 55 |  | $ 54
Number of shares sold (shares) | 193 | 49 |  |  |
Money Market Funds |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Share price (per share amount) |  |  | $ 1.00 |  |

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