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Investments (Other Investments) (Details) $ in Billions | Jun. 30, 2018 USD ($) preferred_stock_series | Dec. 31, 2017 USD ($)
--- | --- | ---
Level 2 [Member] | AirTouch [Member] |  |
Schedule of Other Investments [Line Items] |  |
Fair value of redeemable preferred shares | $ 1.7 |
AirTouch [Member] |  |
Schedule of Other Investments [Line Items] |  |
Number of series of preferred stock | preferred_stock_series | 2 |
Held to maturity investment | $ 1.6 | $ 1.6
AirTouch [Member] | Level 2 [Member] |  |
Schedule of Other Investments [Line Items] |  |
Fair value of held to maturity investment redeemable preferred shares | $ 1.7 |

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