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	$ 329	$ 281	$ 322	$ 268
Costs incurred	43	47	100	99
Amortization	(51)	(41)	(101)	(80)
End of period balance	321	287	321	287
Contract Fulfillment Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	198	150	187	133
Costs incurred	44	29	78	58
Amortization	(35)	(18)	(58)	(30)
End of period balance	$ 207	$ 161	$ 207	$ 161