Supplemental Financial Information (Components of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized gains (losses) on marketable securities	$ (43)		$ 2
Deferred gains (losses) on cash flow hedges	(12)		(52)
Unrecognized gains (losses) on employee benefit obligations	270		8
Cumulative translation adjustments	166		76
Accumulated other comprehensive income (loss), net of deferred taxes	381	$ 98	34
NBCUniversal Media LLC [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized gains (losses) on marketable securities	(233)		0
Deferred gains (losses) on cash flow hedges	(4)		(8)
Unrecognized gains (losses) on employee benefit obligations	115		3
Cumulative translation adjustments	93		121
Accumulated other comprehensive income (loss), net of deferred taxes	$ (29)	$ (135)	$ 116