Investments (Equity Method Investments Summarized Financial Information) (Details) - NBCUniversal Media LLC [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Current assets	$ 2,612	$ 2,105
Noncurrent assets	3,690	2,724
Current liabilities	2,205	1,921
Noncurrent liabilities	3,969	2,853
Revenue	4,234	4,285	$ 3,944
Operating income (loss)	(465)	(182)	(1,609)
Net income (loss)	$ (656)	$ (313)	$ (1,820)