Condensed Consolidating Financial Information (Narrative) (Details) - Sep. 30, 2017 $ in Millions	JPY (¥)	USD ($)
NBCUniversal Enterprise Senior Debt Securities [Member]
Debt Instrument [Line Items]
Principal amount of debt securities subject to guarantee		$ 4,800
NBCUniversal Enterprise Credit Facility [Member]
Debt Instrument [Line Items]
Revolving credit facility subject to guarantee		1,500
Universal Studios Japan Term Loan Maturing March 2022 [Member]
Debt Instrument [Line Items]
Term loans subject to guarantee | ¥	¥ 450,000,000,000
Comcast Holdings' ZONES due October 2029 [Member]
Debt Instrument [Line Items]
Principal amount of debt securities subject to guarantee		185
Comcast Holdings' ZONES due November 2029 [Member]
Debt Instrument [Line Items]
Principal amount of debt securities not subject to guarantee		$ 62