Revenue Recognition - Capitalized Contract Costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Contract Acquisition Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	$ 90	$ 90	$ 90	$ 91
Costs incurred	13	14	29	28
Amortization	(16)	(16)	(32)	(31)
End of period balance	87	88	87	88
Contract Fulfillment Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	57	60	57	61
Costs incurred	15	3	19	7
Amortization	(12)	(5)	(16)	(10)
End of period balance	$ 60	$ 58	$ 60	$ 58