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Fair Value Measurements (Equity Method Investments, Summarized Financial Information) (Details) shares in Millions, $ in Millions | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2018 USD ($) | Dec. 31, 2018 USD ($) shares | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2018 R / shares | Dec. 31, 2016 R / shares
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Impairment charge | $ 83 |  |  |  |  |
Carrying value of equity method investments |  | $ 287 | $ 405 |  |  |
Other investments |  | 75 | 56 |  |  |
Unrealized investment (loss) income |  | 0 | (7) | $ 21 |  |
Alexander Forbes Group Holdings Limited |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Carrying value of equity method investments |  | $ 144 |  |  |  |
Equity investment shares owned by the Company (in shares) | shares |  | 443 |  |  |  |
Share price | R / shares |  |  |  |  | R 5.14 |
Market value of equity method investment |  | $ 159 |  |  |  |
Alexander Forbes Group Holdings Limited | Mercer Consulting Group |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Percentage of interest acquired |  |  |  |  |  | 33.00%
Share price | R / shares |  |  |  |  |  | R 7.50
Benefitfocus |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Unrealized investment (loss) income |  | 54 |  |  |  |
Private Equity Funds |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Carrying value of equity method investments |  | 82 | 76 |  |  |
Investment (loss) income |  | 16 | 14 |  |  |
Corporate stocks |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Investments, fair value |  | 146 | 100 |  |  |
Other Investments |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Unrealized investment (loss) income |  | $ 1 | $ 1 |  |  |

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