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Investments (Marketable Equity Securities) (Details) - USD ($) $ in Millions |  | 1 Months Ended | 3 Months Ended | 6 Months Ended |
--- | --- | --- | --- | --- | ---
Mar. 31, 2017 | Mar. 31, 2017 | Jun. 30, 2018 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017
Schedule of Marketable Equity Securities [Line Items] |  |  |  |  |  |
Marketable equity securities |  |  | $ 451 | $ 451 |  | $ 433
NBCUniversal Media LLC [Member] |  |  |  |  |  |
Schedule of Marketable Equity Securities [Line Items] |  |  |  |  |  |
Contribution from member |  |  |  |  | $ 662 |
Marketable equity securities |  |  | 385 | 385 |  | 430
Snap [Member] |  |  |  |  |  |
Schedule of Marketable Equity Securities [Line Items] |  |  |  |  |  |
Payments to acquire marketable securities |  | $ 500 |  |  |  |
Marketable equity securities |  |  | 385 | 385 |  | 430
Marketable securities, recognized unrealized gains (losses) |  |  | (82) | (45) |  |
Snap [Member] | NBCUniversal Media LLC [Member] |  |  |  |  |  |
Schedule of Marketable Equity Securities [Line Items] |  |  |  |  |  |
Contribution from member | $ 662 |  |  |  |  |
Marketable equity securities |  |  | 385 | 385 |  | $ 430
Marketable securities, recognized unrealized gains (losses) |  |  | $ (82) | $ (45) |  |

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