Revenue Recognition - Capitalized Contract Costs (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Capitalized Contract Cost [Roll Forward]
Beginning of period balance		$ 0
End of period balance	$ 52	52
Contract Acquisition Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	91	91
Costs incurred	14	29
Amortization	(16)	(31)
End of period balance	89	89
Contract Fulfillment Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	35	37
Costs incurred	3	6
Amortization	(5)	(10)
End of period balance	$ 33	$ 33