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Fair Value Measurements (Narrative) (Details) $ / shares in Units, shares in Millions, $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Apr. 30, 2019 USD ($) | Mar. 31, 2019 USD ($) | Sep. 30, 2019 USD ($) $ / shares shares | Jun. 30, 2019 USD ($) | Mar. 31, 2019 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2019 USD ($) $ / shares shares | Sep. 30, 2018 USD ($) | Sep. 30, 2019 R / shares | Dec. 31, 2018 USD ($) | Sep. 30, 2018 R / shares | Dec. 31, 2014 R / shares
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net charge for adjustments related to acquisition related accounts |  |  |  |  |  |  | $ 26 | $ 19 |  |  |  |
Increase in fair value of contingent consideration due to 5% increase in projections |  |  |  |  |  |  | 46 |  |  |  |  |
Decrease in fair value of contingent consideration due to 5% decrease in projections |  |  |  |  |  |  | 39 |  |  |  |  |
Carrying value of investment |  |  | $ 452 |  |  |  | 452 |  |  | $ 287 |  |
Equity method income |  |  | 3 |  |  | $ 4 | 10 | 14 |  |  |  |
Equity securities |  |  | 19 |  |  |  | 19 |  |  | 146 |  |
Gains on equity securities |  |  | 4 |  |  | 25 | 10 | 43 |  |  |  |
Equity investments without readily determinable market value |  |  | 47 |  |  |  | $ 47 |  |  | 75 |  |
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 |  |  | 219 |  |  |  | $ 219 |  |  | 61 |  |
Private Equity Funds, Non-US |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Carrying value of investment |  |  | 95 |  |  |  | 95 |  |  | 82 |  |
Alexander Forbes Group Holdings Limited |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share price (per share amount) | R / shares |  |  |  |  |  |  |  |  | R 5.6 |  | R 4.95 | R 7.50
Carrying value of investment |  |  | $ 138 |  |  |  | $ 138 |  |  |  |  |
Percentage of ownership in equity investment |  |  | 33.00% |  |  |  | 33.00% |  |  |  |  |
Other than temporary impairment |  |  |  |  |  | $ 81 |  | $ 81 |  |  |  |
Shares owned in investment (in shares) | shares |  |  | 443 |  |  |  | 443 |  |  |  |  |
Equity method investment, quoted market value |  |  | $ 166 |  |  |  | $ 166 |  |  |  |  |
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 |  |  |  |  |  |  |  |
Treasury Lock | Jardine Lloyd Thompson Group plc |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Derivative hedges, assets |  |  |  |  |  |  |  |  |  | 2,000 |  |
Money Market Funds |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share price (per share amount) | $ / shares |  |  | $ 1.00 |  |  |  | $ 1.00 |  |  |  |  |
Fair Value, Measurements, Recurring | Other Assets |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Contingent purchase consideration asset |  |  | $ 65 |  |  |  | $ 65 |  |  | 0 |  |
Equity securities |  |  | 5 |  |  |  | 5 |  |  | 133 |  |
Fair Value, Measurements, Recurring | Other Assets | Unobservable Inputs (Level 3) |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Contingent purchase consideration asset |  |  | 65 |  |  |  | 65 |  |  | 0 |  |
Equity securities |  |  | $ 0 |  |  |  | $ 0 |  |  | $ 0 |  |

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