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Fair Value Measurements (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions |  | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | --- | ---
Sep. 18, 2018 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Total consideration |  |  |  | $ 661 | $ 734 |  |
Adjustments to acquisition related contingent consideration liability |  |  |  | 19 |  |  |
Increase in fair value of contingent consideration due to 5% increase in projections |  |  |  | 41 |  |  |
Decrease in fair value of contingent consideration due to 5% decrease in projections |  |  |  | (31) |  |  |
Carrying value of investment |  | $ 296 |  | 296 |  | $ 405 |
Unrealized investment (losses) gains |  | 0 | $ (8) | 0 | $ 11 |  |
Other investments |  | 70 |  | $ 70 |  |  |
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 |  | 60 |  | $ 60 |  | 63 |
Private Equity Funds, Non-US |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Carrying value of investment |  | 83 |  | 83 |  | $ 76 |
Equity Securities |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Investments, fair value |  | $ 132 |  | $ 132 |  |  |
Alexander Forbes Group Holdings Limited |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Share price (per share amount) |  | $ 4.95 |  | $ 4.95 |  |  | $ 7.50
Carrying value of investment |  | $ 153 |  | $ 153 |  |  |
Other than temporary impairment |  |  |  | $ 81 |  |  |
Percentage of ownership in equity investment |  | 33.00% |  | 33.00% |  |  |
Benefitfocus |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Unrealized investment (losses) gains |  | $ 6 |  | $ 38 |  |  |
Money Market Funds |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Share price (per share amount) |  | $ 1.00 |  | $ 1.00 |  |  |
Jardine Lloyd Thompson Group plc |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Total consideration | $ 5,200 |  |  |  |  |  |

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