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Fair Value Measurements (Narrative) (Details) $ / shares in Units, shares in Millions, $ in Millions | 3 Months Ended | 6 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Jun. 30, 2018 USD ($) $ / shares shares | Jun. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) $ / shares shares | Jun. 30, 2017 USD ($) | Jun. 30, 2018 R / shares | Dec. 31, 2017 USD ($) | Dec. 31, 2014 R / shares
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Adjustments to acquisition related contingent consideration liability |  |  | $ 11 |  |  |  |
Increase in fair value of contingent consideration due to 5% increase in projections |  |  | 22 |  |  |  |
Decrease in fair value of contingent consideration due to 5% decrease in projections |  |  | (23) |  |  |  |
Carrying value of investment | $ 391 |  | 391 |  |  | $ 405 |
Unrealized investment gains | 0 | $ 24 | 0 | $ 19 |  |  |
Other investments | 64 |  | $ 64 |  |  |  |
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 | 63 |  | $ 63 |  |  | 63 |
Private Equity Funds, Non-US |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Carrying value of investment | 82 |  | 82 |  |  | $ 76 |
Equity Securities |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Investments, fair value | 113 |  | 113 |  |  |  |
Alexander Forbes Group Holdings Limited |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Share price (per share amount) | R / shares |  |  |  |  | R 5.77 |  | R 7.50
Carrying value of investment | $ 246 |  | $ 246 |  |  |  |
Percentage of ownership in equity investment | 33.00% |  | 33.00% |  |  |  |
Investment shares owned | shares | 443 |  | 443 |  |  |  |
Market value of investment | $ 188 |  | $ 188 |  |  |  |
Share price, weighted average (per share amount) | R / shares |  |  |  |  | R 6.66 |  |
Share price, percentage of cost basis | 89.00% |  | 89.00% |  |  |  |
Alexander Forbes Group Holdings Limited | Minimum |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Share price (per share amount) | $ / shares | $ 5.00 |  | $ 5.00 |  |  |  |
Share price, percentage of purchase price | 67.00% |  | 67.00% |  |  |  |
Alexander Forbes Group Holdings Limited | Maximum |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Share price (per share amount) | $ / shares | $ 7.34 |  | $ 7.34 |  |  |  |
Share price, percentage of purchase price | 98.00% |  | 98.00% |  |  |  |
Benefitfocus |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Unrealized investment gains |  |  | $ 19 |  |  |  |
Money Market Funds |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Share price (per share amount) | $ / shares | $ 1.00 |  | $ 1.00 |  |  |  |

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