Condensed Consolidating Financial Information (Condensed Consolidating 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
Revenue:
Service revenue									$ 84,526	$ 80,403	$ 74,510
Management fee revenue									0	0	0
Total revenue	$ 21,915	$ 20,983	$ 21,165	$ 20,463	$ 21,025	$ 21,319	$ 19,269	$ 18,790	84,526	80,403	74,510
Costs and Expenses:
Programming and production									25,384	24,463	22,550
Other operating and administrative									25,013	23,416	21,325
Advertising, marketing and promotion									6,317	6,107	5,957
Depreciation									7,914	7,464	6,781
Amortization									2,353	2,094	1,899
Other operating gains									(442)	0	0
Total costs and expenses									66,539	63,544	58,512
Operating income	4,107	4,792	4,558	4,530	4,264	4,440	4,066	4,089	17,987	16,859	15,998
Other Income (Expense):
Interest expense									(3,086)	(2,942)	(2,702)
Investment income (loss), net									253	213	81
Equity in net income (losses) of investees, net									107	(104)	(325)
Other income (expense), net									61	327	320
Total other income (expense)									(2,665)	(2,506)	(2,626)
Income before income taxes									15,322	14,353	13,372
Income tax (expense) benefit									7,578	(5,308)	(4,959)
Net income									22,900	9,045	8,413
Net (income) loss attributable to noncontrolling interests and redeemable subsidiary preferred stock									(186)	(350)	(250)
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
Comprehensive income attributable to Parent									22,801	8,967	8,135
Elimination and Consolidation Adjustments [Member]
Revenue:
Service revenue									0	0	0
Management fee revenue									(2,237)	(2,116)	(1,982)
Total revenue									(2,237)	(2,116)	(1,982)
Costs and Expenses:
Programming and production									0	0	0
Other operating and administrative									(2,237)	(2,116)	(1,982)
Advertising, marketing and promotion									0	0	0
Depreciation									0	0	0
Amortization									0	0	0
Other operating gains									0	0	0
Total costs and expenses									(2,237)	(2,116)	(1,982)
Operating income									0	0	0
Other Income (Expense):
Interest expense									0	0	0
Investment income (loss), net									0	0	0
Equity in net income (losses) of investees, net									(77,072)	(37,554)	(34,116)
Other income (expense), net									0	0	0
Total other income (expense)									(77,072)	(37,554)	(34,116)
Income before income taxes									(77,072)	(37,554)	(34,116)
Income tax (expense) benefit									0	0	0
Net income									(77,072)	(37,554)	(34,116)
Net (income) loss attributable to noncontrolling interests and redeemable subsidiary preferred stock									0	0	0
Net Income Attributable to Comcast Corporation									(77,072)	(37,554)	(34,116)
Comprehensive income attributable to Parent									(76,909)	(37,769)	(34,042)
Comcast Parent [Member] | Reportable Legal Entities [Member]
Revenue:
Service revenue									0	0	0
Management fee revenue									1,128	1,067	1,005
Total revenue									1,128	1,067	1,005
Costs and Expenses:
Programming and production									0	0	0
Other operating and administrative									766	813	760
Advertising, marketing and promotion									0	0	0
Depreciation									31	28	31
Amortization									6	6	6
Other operating gains									0	0	0
Total costs and expenses									803	847	797
Operating income									325	220	208
Other Income (Expense):
Interest expense									(2,172)	(1,941)	(1,744)
Investment income (loss), net									(1)	7	6
Equity in net income (losses) of investees, net									24,056	9,809	9,159
Other income (expense), net									0	0	(3)
Total other income (expense)									21,883	7,875	7,418
Income before income taxes									22,208	8,095	7,626
Income tax (expense) benefit									506	600	537
Net income									22,714	8,695	8,163
Net (income) loss attributable to noncontrolling interests and redeemable subsidiary preferred stock									0	0	0
Net Income Attributable to Comcast Corporation									22,714	8,695	8,163
Comprehensive income attributable to Parent									22,801	8,967	8,135
Subsidiary Issuer [Member] | Reportable Legal Entities [Member] | Comcast Holdings [Member]
Revenue:
Service revenue									0	0	0
Management fee revenue									0	0	0
Total revenue									0	0	0
Costs and Expenses:
Programming and production									0	0	0
Other operating and administrative									0	0	0
Advertising, marketing and promotion									0	0	0
Depreciation									0	0	0
Amortization									0	0	0
Other operating gains									0	0	0
Total costs and expenses									0	0	0
Operating income									0	0	0
Other Income (Expense):
Interest expense									(12)	(12)	(12)
Investment income (loss), net									109	(5)	(1)
Equity in net income (losses) of investees, net									21,622	9,286	8,651
Other income (expense), net									0	0	0
Total other income (expense)									21,719	9,269	8,638
Income before income taxes									21,719	9,269	8,638
Income tax (expense) benefit									156	6	4
Net income									21,875	9,275	8,642
Net (income) loss attributable to noncontrolling interests and redeemable subsidiary preferred stock									0	0	0
Net Income Attributable to Comcast Corporation									21,875	9,275	8,642
Comprehensive income attributable to Parent									21,873	9,317	8,625
Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member] | CCCL Parent [Member]
Revenue:
Service revenue									0	0	0
Management fee revenue									1,109	1,049	977
Total revenue									1,109	1,049	977
Costs and Expenses:
Programming and production									0	0	0
Other operating and administrative									1,109	1,049	977
Advertising, marketing and promotion									0	0	0
Depreciation									0	0	0
Amortization									0	0	0
Other operating gains									0	0	0
Total costs and expenses									1,109	1,049	977
Operating income									0	0	0
Other Income (Expense):
Interest expense									(207)	(239)	(270)
Investment income (loss), net									0	0	0
Equity in net income (losses) of investees, net									19,570	8,679	8,040
Other income (expense), net									0	0	0
Total other income (expense)									19,363	8,440	7,770
Income before income taxes									19,363	8,440	7,770
Income tax (expense) benefit									71	84	94
Net income									19,434	8,524	7,864
Net (income) loss attributable to noncontrolling interests and redeemable subsidiary preferred stock									0	0	0
Net Income Attributable to Comcast Corporation									19,434	8,524	7,864
Comprehensive income attributable to Parent									19,437	8,530	7,864
Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member] | NBCUniversal Media Parent [Member]
Revenue:
Service revenue									0	0	0
Management fee revenue									0	0	0
Total revenue									0	0	0
Costs and Expenses:
Programming and production									0	0	0
Other operating and administrative									1,044	932	922
Advertising, marketing and promotion									0	0	0
Depreciation									0	0	0
Amortization									0	0	0
Other operating gains									0	0	0
Total costs and expenses									1,044	932	922
Operating income									(1,044)	(932)	(922)
Other Income (Expense):
Interest expense									(456)	(456)	(462)
Investment income (loss), net									(197)	(25)	(19)
Equity in net income (losses) of investees, net									6,740	5,545	4,852
Other income (expense), net									45	116	(31)
Total other income (expense)									6,132	5,180	4,340
Income before income taxes									5,088	4,248	3,418
Income tax (expense) benefit									(4)	(13)	(4)
Net income									5,084	4,235	3,414
Net (income) loss attributable to noncontrolling interests and redeemable subsidiary preferred stock									0	0	0
Net Income Attributable to Comcast Corporation									5,084	4,235	3,414
Comprehensive income attributable to Parent									5,058	4,312	3,361
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Revenue:
Service revenue									84,526	80,403	74,510
Management fee revenue									0	0	0
Total revenue									84,526	80,403	74,510
Costs and Expenses:
Programming and production									25,384	24,463	22,550
Other operating and administrative									24,331	22,738	20,648
Advertising, marketing and promotion									6,317	6,107	5,957
Depreciation									7,883	7,436	6,750
Amortization									2,347	2,088	1,893
Other operating gains									(442)	0	0
Total costs and expenses									65,820	62,832	57,798
Operating income									18,706	17,571	16,712
Other Income (Expense):
Interest expense									(239)	(294)	(214)
Investment income (loss), net									342	236	95
Equity in net income (losses) of investees, net									5,191	4,131	3,089
Other income (expense), net									16	211	354
Total other income (expense)									5,310	4,284	3,324
Income before income taxes									24,016	21,855	20,036
Income tax (expense) benefit									6,849	(5,985)	(5,590)
Net income									30,865	15,870	14,446
Net (income) loss attributable to noncontrolling interests and redeemable subsidiary preferred stock									(186)	(350)	(250)
Net Income Attributable to Comcast Corporation									30,679	15,520	14,196
Comprehensive income attributable to Parent									$ 30,541	$ 15,610	$ 14,192