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Recent Accounting Pronouncements and Tax Reform (Effect of Changes in Revenue Recognition - Statement of Income) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Revenue $ 21,735 $ 21,286 $ 44,526 $ 41,873
Total costs and expenses 16,721 16,718 34,867 32,763
Operating income 5,014 4,568 9,659 9,110
Net Income Attributable to Comcast Corporation 3,216 2,521 6,334 5,094
NBCUniversal Media LLC [Member]        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Revenue 8,313 8,318 17,843 16,171
Total costs and expenses 6,706 6,764 14,461 13,106
Operating income 1,607 1,554 3,382 3,065
Net Income Attributable to Comcast Corporation $ 1,247 1,279 $ 2,760 2,481
Previous Reported [Member]        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Revenue   21,165   41,628
Total costs and expenses   16,607   32,540
Operating income   4,558   9,088
Net Income Attributable to Comcast Corporation   2,513   5,079
Previous Reported [Member] | NBCUniversal Media LLC [Member]        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Revenue   8,331   16,199
Total costs and expenses   6,780   13,139
Operating income   1,551   3,060
Net Income Attributable to Comcast Corporation   1,276   2,476
Effects of Adoption [Member] | Accounting Standards Update 2014-09 [Member]        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Revenue   121   245
Total costs and expenses   111   223
Operating income   10   22
Net Income Attributable to Comcast Corporation   8   15
Effects of Adoption [Member] | Accounting Standards Update 2014-09 [Member] | NBCUniversal Media LLC [Member]        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Revenue   (13)   (28)
Total costs and expenses   (16)   (33)
Operating income   3   5
Net Income Attributable to Comcast Corporation   $ 3   $ 5