Revenue Recognition - Capitalized Contract Costs (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Contract Acquisition Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	$ 89	$ 91
Costs incurred	29	45
Amortization	(29)	(47)
End of period balance	89	89
Contract Fulfillment Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	58	61
Costs incurred	13	20
Amortization	(13)	(23)
End of period balance	$ 58	$ 58