Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ 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
Earnings Per Share [Abstract]
Net Income Attributable to Comcast Corporation	$ 14,985	$ 2,650	$ 2,513	$ 2,566	$ 2,296	$ 2,237	$ 2,028	$ 2,134	$ 22,714	$ 8,695	$ 8,163
Basic EPS attributable to Comcast Corporation shareholders (in shares)									4,708	4,819	4,972
Basic earnings per common share attributable to Comcast Corporation shareholders (in dollars per share)	$ 3.22	$ 0.56	$ 0.53	$ 0.54	$ 0.48	$ 0.47	$ 0.42	$ 0.44	$ 4.82	$ 1.80	$ 1.64
Effect of dilutive securities:
Net Income Attributable to Comcast Corporation	$ 14,985	$ 2,650	$ 2,513	$ 2,566	$ 2,296	$ 2,237	$ 2,028	$ 2,134	$ 22,714	$ 8,695	$ 8,163
Assumed exercise or issuance of shares relating to stock plans (in shares)									78	56	63
Diluted EPS attributable to Comcast Corporation shareholders (in shares)									4,786	4,875	5,035
Diluted earnings per common share attributable to Comcast Corporation shareholders (in dollars per share)	$ 3.17	$ 0.55	$ 0.52	$ 0.53	$ 0.48	$ 0.46	$ 0.41	$ 0.43	$ 4.75	$ 1.78	$ 1.62