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Recent Accounting Pronouncements and Tax Reform (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017 | Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Income tax benefit, 2017 tax reform legislation |  | $ 12,700 |
Income tax benefit, 2018 tax reform legislation | $ 128 |  |
Cumulative Effect of New Accounting Principle in Period of Adoption |  | 33 |
NBCUniversal Media LLC [Member] |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption |  | 0 |
Member's Capital [Member] | NBCUniversal Media LLC [Member] |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption |  | (232) |
AOCI Attributable to Parent [Member] |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption |  | 76 |
AOCI Attributable to Parent [Member] | NBCUniversal Media LLC [Member] |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption |  | $ 232 |
Accounting Standards Update 2016-01 [Member] | Member's Capital [Member] | NBCUniversal Media LLC [Member] |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption |  |  | $ (232)
Accounting Standards Update 2016-01 [Member] | AOCI Attributable to Parent [Member] | NBCUniversal Media LLC [Member] |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption |  |  | $ 232

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