v3.8.0.1

Short-term and Long-term Investments, Restricted Deposits - Narrative (Details) shares in Thousands, $ in Millions | 1 Months Ended | 12 Months Ended
--- | --- | ---
Jan. 31, 2016 USD ($) shares | Dec. 31, 2017 USD ($) position | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |  |
Percentage of voting interests acquired | 19.00% |  |  |
Stock issued, shares (in shares) | shares | 1,144 |  |  |
Stock issued, value | $ | $ 68 |  |  |
Positions from which gross unrealized losses were generated | position |  | 1,979 |  |
Total number of positions | position |  | 3,475 |  |
Income from equity method investments | $ |  | $ 3 | $ 5 | $ 8
Commercial mortgage- backed securities |  |  |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |  |
Investments that carry rating of A A plus, weighted average, duration |  | 4 years |  |
External Credit Rating, Investment Grade | Rated Securities |  |  |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |  |
Percentage of investment grade investments rated by nationally recognized statistical rating organizations |  | 96.00% |  |

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