Commitments and Contingencies (Rental expense) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitment And Contingencies [Line Items]
Rent expense	$ 839	$ 744	$ 608
NBCUniversal Media LLC [Member]
Commitment And Contingencies [Line Items]
Rent expense	$ 274	$ 259	$ 213

Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line.

No definition available.

Name:	us-gaap_LeaseAndRentalExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration