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Investments (Marketable Equity Securities) (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018
Schedule of Marketable Equity Securities [Line Items] |  |  |
Marketable equity securities | $ 538 |  | $ 341
NBCUniversal Media LLC [Member] |  |  |
Schedule of Marketable Equity Securities [Line Items] |  |  |
Marketable equity securities | 324 |  | 162
Snap [Member] |  |  |
Schedule of Marketable Equity Securities [Line Items] |  |  |
Marketable securities, recognized unrealized gains (losses) | 162 | $ 37 |
Marketable equity securities | 324 |  | 162
Snap [Member] | NBCUniversal Media LLC [Member] |  |  |
Schedule of Marketable Equity Securities [Line Items] |  |  |
Marketable securities, recognized unrealized gains (losses) | 162 | $ 37 |
Marketable equity securities | $ 324 |  | $ 162

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