Investments (Cost Method Investments) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) preferred_stock_series	Dec. 31, 2016 USD ($)	Nov. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Air Touch [Member]
Schedule of Cost-method Investments [Line Items]
Number of series of preferred stock | preferred_stock_series	3
Preferred stock aggregate redemption value	$ 1,750
Redeemable subsidiary preferred shares	1,600	$ 1,600
Nonredeemable subsidiary preferred shares	100	100
Level 2 [Member] | Air Touch [Member]
Schedule of Cost-method Investments [Line Items]
Fair value of redeemable preferred shares	$ 1,700	1,700
Air Touch [Member]
Schedule of Cost-method Investments [Line Items]
Number of series of preferred stock | preferred_stock_series	2
Preferred stock aggregate redemption value	$ 1,650
Air Touch [Member] | Level 2 [Member]
Schedule of Cost-method Investments [Line Items]
Fair value of cost method Investment redeemable preferred shares	$ 1,700	$ 1,700
Vox Media [Member]
Schedule of Cost-method Investments [Line Items]
Purchases of investments				$ 200
Vox Media [Member] | NBCUniversal Media LLC [Member]
Schedule of Cost-method Investments [Line Items]
Purchases of investments				200
BuzzFeed [Member]
Schedule of Cost-method Investments [Line Items]
Purchases of investments			$ 200	200
BuzzFeed [Member] | NBCUniversal Media LLC [Member]
Schedule of Cost-method Investments [Line Items]
Purchases of investments			$ 200	$ 200