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Fair Value Measurements (Narrative) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended |
--- | --- | --- | --- | ---
May 31, 2020 | Feb. 29, 2020 | Apr. 30, 2019 | Mar. 31, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Increase in fair value of contingent consideration due to 5% increase in projections |  |  |  |  |  |  |  |  | $ 9 |  |
Decrease in fair value of contingent consideration due to 5% decrease in projections |  |  |  |  |  |  |  |  | 21 |  |
Net charge (credit) for adjustments related to acquisition related accounts |  |  |  |  |  |  |  |  | 8 |  |
Carrying value of investment |  |  |  |  | $ 265 |  |  |  | 265 |  | $ 434
Equity securities |  |  |  |  | 58 |  |  |  | 58 |  | 19
Gain (loss) on equity securities |  |  |  |  | (16) | $ 4 |  |  | (19) | $ 10 |
Equity investments without readily determinable market value |  |  |  |  | 43 |  |  |  | $ 43 |  | 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 |  |  |  |  | 169 |  |  |  | $ 169 |  | 183
Private Equity Funds |  |  |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Carrying value of investment |  |  |  |  | 96 |  |  |  | 96 |  | $ 107
Equity method income (loss) |  |  |  |  | 2 | $ 3 |  |  | (6) | $ 10 |
Alexander Forbes Group Holdings Limited |  |  |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Equity method investment, shares owned |  |  |  |  |  |  |  |  |  |  | 443
Number of shares sold (shares) | 193 | 49 |  |  |  |  |  |  |  |  |
Investments, fair value |  |  |  |  | $ 41 |  |  |  | 41 |  |
BenefitFocus |  |  |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from sale of other investments |  |  | $ 17 | $ 132 |  |  |  | $ 115 |  |  |
Payscale |  |  |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from sale of other investments |  |  |  |  |  |  | $ 47 |  |  |  |
Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Change in contingent purchase consideration |  |  |  |  |  |  |  |  | 14 |  |
Increase in fair value of contingent consideration due to 5% increase in projections |  |  |  |  |  |  |  |  | 6 |  |
Decrease in fair value of contingent consideration due to 5% decrease in projections |  |  |  |  |  |  |  |  | $ 6 |  |
Money Market Funds |  |  |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Share price (per share amount) |  |  |  |  | $ 1.00 |  |  |  | $ 1.00 |  |
Alexander Forbes Group Holdings Limited |  |  |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment owned, shares |  |  |  |  | 201 |  |  |  | 201 |  |

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