Supplemental Financial Information Supplemental Financial Information (Components of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized gains (losses) on marketable securities	$ 3	$ (43)
Deferred gains (losses) on cash flow hedges	55	9
Unrecognized gains (losses) on employee benefit obligations	325	260
Cumulative translation adjustments	(751)	153
Accumulated other comprehensive income (loss)	(368)	379
NBCUniversal Media LLC [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized gains (losses) on marketable securities	0	(233)
Deferred gains (losses) on cash flow hedges	12	10
Unrecognized gains (losses) on employee benefit obligations	140	126
Cumulative translation adjustments	102	77
Accumulated other comprehensive income (loss)	$ 254	$ (20)

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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Accumulated change, net of tax, in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of cash flow hedges. Includes an entity's share of an equity investee's Increase or Decrease in deferred hedging gains or losses.

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Amount, after tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Accumulated adjustment, net of tax, that results from the process of translating subsidiary financial statements and foreign equity investments into the reporting currency from the functional currency of the reporting entity, net of reclassification of realized foreign currency translation gains or losses.

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