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Related Party Transactions (Narrative) (Details) - USD ($) |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Mar. 01, 2018 | Oct. 31, 2017 | Sep. 30, 2019 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2016
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Repayments of Debt |  |  |  | $ 14,354,000,000 | $ 8,798,000,000 | $ 6,364,000,000 |
Other noncurrent liabilities |  |  |  | 16,765,000,000 | 15,329,000,000 |  |
Recognized share-based compensation expense |  |  |  | 825,000,000 | 639,000,000 | 586,000,000 |
Restricted share units [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Recognized share-based compensation expense |  |  |  | 564,000,000 | 402,000,000 | 349,000,000 |
Unrecognized pretax compensation expense on nonvested awards |  |  |  | $ 1,100,000,000 |  |  |
Unrecognized pretax compensation expense on nonvested awards, weighted average period of recognition (in years) |  |  |  | 1 year 6 months |  |  |
Stock options [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Recognized share-based compensation expense |  |  |  | $ 231,000,000 | 205,000,000 | 205,000,000 |
Unrecognized pretax compensation expense on nonvested awards |  |  |  | $ 493,000,000 |  |  |
Unrecognized pretax compensation expense on nonvested awards, weighted average period of recognition (in years) |  |  |  | 1 year 8 months 12 days |  |  |
NBCUniversal Media LLC [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Note receivable from Comcast |  |  |  | $ 1,069,000,000 | 0 |  |
Repayments of Debt |  |  |  | 3,778,000,000 | 438,000,000 | 3,498,000,000 |
Other noncurrent liabilities |  |  |  | 6,548,000,000 | 5,177,000,000 |  |
Recognized share-based compensation expense |  |  |  | 188,000,000 | 151,000,000 | 133,000,000 |
NBCUniversal Media LLC [Member] | Restricted share units [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Unrecognized pretax compensation expense on nonvested awards |  |  |  | $ 308,000,000 |  |  |
Unrecognized pretax compensation expense on nonvested awards, weighted average period of recognition (in years) |  |  |  | 1 year 6 months |  |  |
NBCUniversal Media LLC [Member] | Stock options [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Unrecognized pretax compensation expense on nonvested awards |  |  |  | $ 13,000,000 |  |  |
Unrecognized pretax compensation expense on nonvested awards, weighted average period of recognition (in years) |  |  |  | 8 months 12 days |  |  |
NBCUniversal Media LLC [Member] | Carrying Value Of Related Party Contractual Obligation [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Other noncurrent liabilities |  |  |  | $ 383,000,000 | 383,000,000 |  |
NBCUniversal Media LLC [Member] | NBCUniversal Holdings Preferred Stock [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Preferred stock, dividend rate |  |  |  | 8.25% |  |  |
Preferred stock liquidation preference value |  |  |  | $ 9,400,000,000 |  |  |
Preferred stock dividend rate, spread on variable rate | 7.44% |  |  |  |  |  |
NBCUniversal Media LLC [Member] | Comcast and NBCUniversal Revolving Credit Agreements [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Revolving credit agreement capacity |  |  |  |  |  |  | $ 5,000,000,000
NBCUniversal Media LLC [Member] | Comcast and Consolidated Subsidiaries [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Note receivable from Comcast |  |  |  | 1,069,000,000 | 0 |  |
Other noncurrent liabilities |  |  |  | 454,000,000 | $ 410,000,000 |  |
Notes Payable [Member] | 4.00% Notes Due 2049 [Member] | NBCUniversal Media LLC [Member] | Comcast and Consolidated Subsidiaries [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Repayments of Debt |  |  |  | $ 610,000,000 |  |  |
Interest rate |  |  |  | 4.00% |  |  |
Gain (loss) on extinguishment of debt |  |  | $ 178,000,000 |  |  |  |
Senior Notes Exchange [Member] | Senior Notes [Member] | Comcast and NBCUniversal Senior Notes [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  | 5,500,000,000 |
Amount of debt exchanged |  |  |  |  |  | $ 3,900,000,000 |
Senior Notes Exchange [Member] | Senior Notes [Member] | Comcast and NBCUniversal Senior Notes [Member] | NBCUniversal Media LLC [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Amount of debt exchanged |  | $ 3,900,000,000 |  |  |  |  |
Senior Notes Exchange [Member] | Senior Notes [Member] | Senior 6.40% Notes Due 2040 [Member] | NBCUniversal Media LLC [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Interest rate |  | 6.40% |  |  |  |  |
Amount of debt exchanged |  | $ 442,000,000 |  |  |  |  |
Senior Notes Exchange [Member] | Senior Notes [Member] | Senior 6.40% Notes Due 2040 [Member] | NBCUniversal Media LLC [Member] | Comcast and Consolidated Subsidiaries [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Gain (loss) on extinguishment of debt |  | 157,000,000 |  |  |  |  |
Senior Notes Exchange [Member] | Notes Payable [Member] | 4.00% Notes Due 2049 [Member] | NBCUniversal Media LLC [Member] | Comcast and Consolidated Subsidiaries [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Aggregate principal amount |  | $ 610,000,000 |  |  |  |  |
Interest rate |  | 4.00% |  |  |  |  |
Non-Interest Bearing Notes Due 2024 [Member] | NBCUniversal Media LLC [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Note receivable from Comcast |  |  |  | $ 1,300,000,000 |  |  |
Comcast And NBCUniveral Revolving Credit Agreements [Member] | NBCUniversal Media LLC [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Repayments of debt |  |  |  | $ 1,000,000,000.0 |  |  |

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