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Fair Value Measurements (Narrative) (Details) $ / shares in Units, shares in Millions, $ in Millions | 1 Months Ended | 3 Months Ended |  |
--- | --- | --- | --- | ---
Apr. 30, 2019 USD ($) | Mar. 31, 2019 USD ($) | Mar. 31, 2020 USD ($) $ / shares shares | Mar. 31, 2019 USD ($) | Mar. 31, 2020 R / shares | Dec. 31, 2019 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |
Net charge for adjustments related to acquisition related accounts |  |  | $ 1 | $ 11 |  |
Increase in fair value of contingent consideration due to 5% increase in projections |  |  | 13 |  |  |
Decrease in fair value of contingent consideration due to 5% decrease in projections |  |  | 17 |  |  |
Carrying value of investment |  |  | 371 |  |  | $ 434
Equity securities |  |  | 18 |  |  | 19
Gain (loss) on equity securities |  |  | (1) | 3 |  |
Equity investments without readily determinable market value |  |  | $ 32 |  |  | 67
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 |  |  | $ 170 |  |  | 183
Private Equity Funds |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |
Carrying value of investment |  |  | 97 |  |  | $ 107
Equity method income |  |  | (1) | 2 |  |
Alexander Forbes Group Holdings Limited |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |
Share price (per share amount) | R / shares |  |  |  |  | R 4.28 |
Carrying value of investment |  |  | $ 105 |  |  |
Shares owned in investment (in shares) | shares |  |  | 394 |  |  |
Equity method investment, quoted market value |  |  | $ 94 |  |  |
Benefitfocus |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |
Proceeds from sale of other investments | $ 17 | $ 132 |  | $ 115 |  |
Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |
Change in contingent purchase consideration |  |  | $ 1 |  |  |
Money Market Funds |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |
Share price (per share amount) | $ / shares |  |  | $ 1.00 |  |  |

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