Revenue from Contracts with Customers - Contract Balances (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Change in Contract with Customer, Asset and Liability [Abstract]
Contract assets included in other current assets	$ 69	$ 69	$ 140
Contract liabilities included in deferred revenue	677	677	$ 718
Change in contract assets included in other current assets		(71)
Change in contracts liabilities included in deferred revenue		(41)
Amounts included in the beginning of period contract liability balance	31	559
Amounts associated with performance obligations satisfied in previous periods	$ 11	$ 11