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Supplemental Financial Information (Components of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized gains (losses) on marketable securities	$ 0		$ 17
Deferred gains (losses) on cash flow hedges	26		(16)
Unrecognized gains (losses) on employee benefit obligations	302		276
Cumulative translation adjustments	133		151
Accumulated other comprehensive income (loss), net of deferred taxes	461	$ 379	428
NBCUniversal Media LLC [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized gains (losses) on marketable securities	0		(139)
Deferred gains (losses) on cash flow hedges	8		1
Unrecognized gains (losses) on employee benefit obligations	119		118
Cumulative translation adjustments	91		76
Accumulated other comprehensive income (loss), net of deferred taxes	$ 218	$ (20)	$ 56

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Accumulated unrealized gain (loss), after tax, on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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