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Fair Value Measurements (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Fair Value Disclosures [Abstract] |  |  |  |
Gain on sale of marketable securities | $ 29 | $ 15 | $ 79 | $ 38

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- Definition Amount of unrealized and realized gain (loss) on investment in marketable security, including other-than-temporary impairment (OTTI). + References No definition available. + Details
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