Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net Income Attributable to Comcast Corporation	$ 2,511	$ 2,886	$ 3,216	$ 3,118	$ 14,999	$ 2,642	$ 2,521	$ 2,573	$ 11,731	$ 22,735	$ 8,678
Basic EPS attributable to Comcast Corporation shareholders (in shares)									4,584	4,708	4,819
Basic earnings per common share attributable to Comcast Corporation shareholders (in dollars per share)	$ 0.55	$ 0.63	$ 0.70	$ 0.67	$ 3.22	$ 0.56	$ 0.53	$ 0.54	$ 2.56	$ 4.83	$ 1.80
Effect of dilutive securities:
Net Income Attributable to Comcast Corporation	$ 2,511	$ 2,886	$ 3,216	$ 3,118	$ 14,999	$ 2,642	$ 2,521	$ 2,573	$ 11,731	$ 22,735	$ 8,678
Assumed exercise or issuance of shares relating to stock plans (in shares)									56	78	56
Diluted EPS attributable to Comcast Corporation shareholders (in shares)									4,640	4,786	4,875
Diluted earnings per common share attributable to Comcast Corporation shareholders (in dollars per share)	$ 0.55	$ 0.62	$ 0.69	$ 0.66	$ 3.17	$ 0.55	$ 0.52	$ 0.53	$ 2.53	$ 4.75	$ 1.78