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Fair Value Measurements (Narrative) (Details) $ / shares in Units, shares in Millions, $ in Millions | 3 Months Ended | 9 Months Ended |  |  |  |  |  |
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Sep. 30, 2017 USD ($) $ / shares shares | Sep. 30, 2016 USD ($) | Sep. 30, 2017 USD ($) $ / shares shares | Sep. 30, 2016 USD ($) | Sep. 30, 2017 ZAR / shares | Apr. 30, 2017 ZAR / shares | Jan. 01, 2017 ZAR / shares | Dec. 31, 2016 USD ($) | Feb. 24, 2015 USD ($) | Dec. 31, 2014 ZAR / shares
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |
Adjustments to acquisition related contingent consideration liability |  |  | $ (3) |  |  |  |  |  |  |
Increase in fair value of contingent consideration due to 5% increase in projections |  |  | 18 |  |  |  |  |  |  |
Decrease in fair value of contingent consideration due to 5% decrease in projections |  |  | 19 |  |  |  |  |  |  |
Carrying value of investment | $ 405 |  | 405 |  |  |  |  | $ 389 |  |
Unrealized investment (losses) gains | (8) | $ 1 | $ 11 | $ 1 |  |  |  |  |  |
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 | 72 |  | $ 72 |  |  |  |  | $ 79 |  |
Alexander Forbes Group Holdings Limited |  |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |
Share price (per share amount) | ZAR / shares |  |  |  |  | ZAR 6.75 |  |  |  |  | ZAR 7.50
Carrying value of investment | $ 268 |  | $ 268 |  |  |  |  |  |  |
Percentage of ownership in equity investment | 33.00% |  | 33.00% |  |  |  |  |  |  |
Investment shares owned | shares | 443 |  | 443 |  |  |  |  |  |  |
Market value of investment | $ 225 |  | $ 225 |  |  |  |  |  |  |
Share price, percentage of cost basis | 90.00% |  | 90.00% |  |  |  |  |  |  |
Share price, weighted average (per share amount) | ZAR / shares |  |  |  |  | ZAR 6.75 |  |  |  |  |
Alexander Forbes Group Holdings Limited | Minimum |  |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |
Share price (per share amount) | (per share) | $ 6.40 |  | $ 6.40 |  |  | ZAR 5.99 |  |  |  |
Share price, percentage of purchase price | 85.00% |  | 85.00% |  |  |  |  |  |  |
Alexander Forbes Group Holdings Limited | Maximum |  |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |
Share price (per share amount) | (per share) | $ 7.28 |  | $ 7.28 |  |  |  | ZAR 7.89 |  |  |
Share price, percentage of purchase price | 97.00% |  | 97.00% |  |  |  |  |  |  |
Benefitfocus |  |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |  |
Market value of investment | $ 95 |  | $ 95 |  |  |  |  |  |  |
Percentage of business interest acquired |  |  |  |  |  |  |  |  | 9.90% |
Total consideration |  |  |  |  |  |  |  |  | $ 75 |
Unrealized investment (losses) gains |  |  | $ 11 |  |  |  |  |  |  |
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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