Revenue Recognition - Capitalized Contract Costs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Contract Acquisition Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	$ 87	$ 89	$ 90	$ 91
Costs incurred	15	29	44	45
Amortization	(15)	(29)	(47)	(47)
End of period balance	87	89	87	89
Contract Fulfillment Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	60	58	57	61
Costs incurred	10	13	29	20
Amortization	(8)	(13)	(24)	(23)
End of period balance	$ 62	$ 58	$ 62	$ 58