Related Party Transactions (Condensed Consolidated Statement of Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Revenue	$ 21,915	$ 20,983	$ 21,165	$ 20,463	$ 21,025	$ 21,319	$ 19,269	$ 18,790	$ 84,526	$ 80,403	$ 74,510
Operating costs and expenses									(66,539)	(63,544)	(58,512)
NBCUniversal Media LLC [Member]
Related Party Transaction [Line Items]
Revenue									32,997	31,593	28,462
Operating costs and expenses									(26,516)	(26,171)	(23,643)
Comcast And Consolidated Subsidiaries [Member] | NBCUniversal Media LLC [Member]
Related Party Transaction [Line Items]
Revenue									1,837	1,742	1,349
Operating costs and expenses									(214)	(220)	(246)
Other income (expense)									$ (250)	$ (69)	$ (37)