Investments (Equity Method Investments) (Details) - USD ($) $ in Millions		1 Months Ended			12 Months Ended
	Jan. 01, 2016	Jul. 31, 2017	Aug. 31, 2016	Jan. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jul. 31, 2016
Schedule of Equity Method Investments [Line Items]
Equity in net income (losses) of investees					$ 107	$ (104)	$ (325)
NBCUniversal Media LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (losses) of investees					(201)	(99)	(376)
Atairos [Member]
Schedule of Equity Method Investments [Line Items]
Term of agreement	12 years
Funding commitment	$ 4,000
Management fee funding commitment	$ 40
Percent of losses	99.00%
Percentage of residual returns	86.50%
Purchases of investments					994	1,200
Equity in net income (losses) of investees					281	(38)
Pretax gain, sale of business		$ 105
Atairos [Member] | Maximum [Member]
Schedule of Equity Method Investments [Line Items]
Funding commitment by management company investors	$ 100
Atairos [Member] | Minimum [Member] | Former CFO [Member]
Schedule of Equity Method Investments [Line Items]
Funding commitment by management company investors	$ 40
Hulu [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (losses) of investees					(276)	(168)	(106)
Cash capital contribution, equity method investment					300	50	123
Ownership percentage			30.00%					33.00%
Put/call period			3 years
Deferred gain			$ 159
Hulu [Member] | NBCUniversal Media LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (losses) of investees					(276)	(168)	(106)
Cash capital contribution, equity method investment					$ 300	$ 50	123
Ownership percentage			30.00%					33.00%
Put/call period			3 years
Deferred gain			$ 159
Hulu [Member] | Time Warner, Inc. [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage			10.00%
Hulu [Member] | Time Warner, Inc. [Member] | NBCUniversal Media LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage			10.00%
The Weather Channel [Member]
Schedule of Equity Method Investments [Line Items]
Pretax gain on sale of investment				$ 108
Goodwill impairment loss from equity method investments							333
The Weather Channel [Member] | NBCUniversal Media LLC [Member]
Schedule of Equity Method Investments [Line Items]
Pretax gain on sale of investment				$ 108
Goodwill impairment loss from equity method investments							$ 333