Revenue Recognition - Capitalized Contract Cost (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	$ 34	$ 13
Costs incurred	16	42
Amortization	(5)	(10)
End of period balance	45	45
Contract Fulfillment Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	52	14
Costs incurred	22	70
Amortization	(8)	(18)
End of period balance	$ 66	$ 66