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Fair Value Measurements (Equity Method Investments, Summarized Financial Information) (Details) $ / shares in Units, shares in Millions, $ in Millions | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2017 USD ($) $ / shares shares | Dec. 31, 2017 R / shares | Dec. 31, 2016 USD ($) | Dec. 31, 2015 R / shares | Feb. 24, 2015 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Carrying value of equity method investments | $ 405 |  | $ 389 |  |
Gain from deconsolidation of subsidiary | 11 |  |  |  |
Alexander Forbes Group Holdings Limited |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Carrying value of equity method investments | $ 266 |  |  |  |
Equity investment shares owned by the Company (in shares) | shares | 443 |  |  |  |
Market value of equity method investment | $ 239 |  |  |  |
Share price | (per share) | $ 7.50 | R 6.87 |  |  |
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 | Mercer Consulting Group |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Percentage of interest acquired |  |  |  |  | 9.90%
Business acquisition, consideration to be transferred, planned acquisition |  |  |  |  | $ 75
Common stock, value, outstanding | $ 76 |  |  |  |
Marsh India Insurance Brokers Limited |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Percentage of ownership in equity method investment | 26.00% |  |  |  |
Local shareholders, ownership percentage | 74.00% |  |  |  |
Minimum | Alexander Forbes Group Holdings Limited |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Share price | $ / shares | $ 5.26 |  |  |  |
Maximum | Alexander Forbes Group Holdings Limited |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Share price | $ / shares | $ 7.95 |  |  |  |

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