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Fair Value Measurements (Narrative) (Details) $ / shares in Units, shares in Millions, $ in Millions | 3 Months Ended |  |  |
--- | --- | --- | --- | ---
Mar. 31, 2018 USD ($) $ / shares shares | Mar. 31, 2017 USD ($) | Mar. 31, 2018 R / shares | Dec. 31, 2017 USD ($) | Dec. 31, 2014 R / shares
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Adjustments to acquisition related contingent consideration liability | $ 5 |  |  |  |
Increase in fair value of contingent consideration due to 5% increase in projections | 19 |  |  |  |
Decrease in fair value of contingent consideration due to 5% decrease in projections | 20 |  |  |  |
Carrying value of investment | 424 |  |  | $ 405 |
Unrealized investment gains (losses) | 0 | $ (5) |  |  |
Other investments | $ 59 |  |  |  |
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 Equity Funds, Foreign |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Carrying value of investment | $ 74 |  |  | $ 76 |
Equity Securities |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Investments, fair value | 89 |  |  |  |
Alexander Forbes Group Holdings Limited |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Share price (per share amount) | R / shares |  |  | R 7.20 |  | R 7.50
Carrying value of investment | $ 286 |  |  |  |
Percentage of ownership in equity investment | 33.00% |  |  |  |
Investment shares owned | shares | 443 |  |  |  |
Market value of investment | $ 269 |  |  |  |
Share price, weighted average (per share amount) | R / shares |  |  | R 6.95 |  |
Share price, percentage of cost basis | 93.00% |  |  |  |
Alexander Forbes Group Holdings Limited | Minimum |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Share price (per share amount) | $ / shares | $ 6.31 |  |  |  |
Share price, percentage of purchase price | 84.00% |  |  |  |
Alexander Forbes Group Holdings Limited | Maximum |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Share price (per share amount) | $ / shares | $ 7.34 |  |  |  |
Share price, percentage of purchase price | 98.00% |  |  |  |
Benefitfocus |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Unrealized investment gains (losses) | $ 8 |  |  |  |
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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