Revenue Recognition - Capitalized Contract Costs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Contract Acquisition Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	$ 90	$ 91
Costs incurred	16	14
Amortization	(16)	(15)
End of period balance	90	90
Contract Fulfillment Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	57	61
Costs incurred	4	4
Amortization	(4)	(5)
End of period balance	$ 57	$ 60