Investments (Equity Method Investments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Equity Method Investments [Line Items]
Equity in net income (losses) of investees	$ (39)	$ (34)	$ 12	$ (64)
Pretax gain, sale of business	105		105
Atairos [Member]
Schedule of Equity Method Investments [Line Items]
Purchases of investments			994
Equity in net income (losses) of investees	7	(9)	106	(36)
Pretax gain, sale of business	105
The Weather Channel [Member]
Schedule of Equity Method Investments [Line Items]
Gain on sale of The Weather Channel product and technology businesses				108
NBCUniversal Media LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (losses) of investees	$ (52)	$ (34)	$ (107)	(55)
NBCUniversal Media LLC [Member] | The Weather Channel [Member]
Schedule of Equity Method Investments [Line Items]
Gain on sale of The Weather Channel product and technology businesses				$ 108